This is a divorce action. The trial court granted child custody to the respondent father. The propriety of that determination is the only issue on appeal.
The parties were married on May 24, 1967. They have two sons, Kody and Kacee Spaulding. The children were eight and *640seven years of age when this action was commenced.
The respondent is a truck driver. He owns at least one truck and hauls bentonite in the Black Hills area where he resides. Occasionally he also does long-distance trucking when local hauling is not available. At the time of trial he had been employed by a trucking firm in Colony, Wyoming. Appellant was not employed prior to the commencement of the divorce but was employed for brief periods during the penden-cy of this action.
Marital difficulties started some time in December, 1976. At that time Marlene asked Jerry Spaulding for a separation and requested that he leave the household. It is unclear precisely when the parties separated, but they were no longer living together as man and wife in July of 1977. In June, 1977, Marlene met a man whom the trial court found moved into the home of plaintiff and the children on or about August 1, 1977. Some time in September appellant left the trailer house in Belle Pourche where she and respondent had lived and moved into a motel room in Custer with her paramour and the children. They later moved to separate residences at Sturgis. The evidence shows that her companion had been convicted of various misdemeanors, but that he seemed to have a good relationship with the children. The trial court found that Marlene had moved three times, that the children had been enrolled in three different school systems since the separation and that the children were not of “tender years,” but rather of an age when they need the association of their father. The custody determination apparently was based largely on the misconduct of the mother in having an illicit affair while she was still married to the defendant. The trial court made findings that this illicit affair was openly conducted in the presence of the children but did not specifically find or conclude that it had a detrimental effect on the children.
Upon review this court must give due regard to the opportunity of the trial court to judge the credibility of the witnesses and its findings will not be set aside unless they are clearly erroneous. SDCL 15-6 — 52(a). The trial court has broad discretion in awarding custody of minor children and we will not interfere with that discretion unless the record presents a clear case of abuse. Holforty v. Holforty, S.D., 272 N.W.2d 810 (1978); Pochop v. Pochop, 89 S.D. 466, 233 N.W.2d 806 (1975); Masek v. Masek, 89 S.D. 62, 228 N.W.2d 334 (1975); Oursland v. Oursland, 83 S.D. 382, 159 N.W.2d 922 (1968); Wiesner v. Wiesner, 80 S.D. 114, 119 N.W.2d 920 (1963); and Howells v. Howells, 79 S.D. 480, 113 N.W.2d 533 (1962).
SDCL 30-27-19 provides that in awarding custody of minor children the court is to be guided by the following considerations:
(1) By what appears to be for the best interests of the child in respect to its temporal and its mental and moral welfare; and if the child be of a sufficient age to form an intelligent preference, the court or judge may consider that preference in determining the question;
(2) As between parents adversely claiming the custody or guardianship, neither parent is entitled to it as of right, but, other things being equal, if the child be of tender years, it should be given to the mother; if it be of an age to require education and preparation for labor or business, then to the father.
As the above statute indicates, the primary consideration is the best interests of the child.1
This court has not had occasion to define precisely what “tender years” means. It appears, however, from an examination of our cases, that children of pre-elementary school age are generally assumed to be of “tender years.” Wiesner v. Wiesner, supra; Septka v. Septka, 80 S.D. 299, 122 N.W.2d 766 (1963); Masek v. Masek, supra; Kester *641v. Kester, S.D., 257 N.W.2d 731 (1977); Hershey v. Hershey, 85 S.D. 85, 177 N.W.2d 267 (1970); and Holforty v. Holforty, supra.2
We agree with decisions in other jurisdictions that children cannot be mechanically defined and whether they are of tender years is subject to considerations such as sex, physical and mental development, home environment, and the changing and evolving structure of the American family. Orezza v. Ramirez, 19 Ariz.App. 405, 507 P.2d 1017 (1973); Denham v. Martina, 214 Cal.App.2d 312, 29 Cal.Rptr. 377 (1963); Russell v. Russell, 20 Cal.App. 457, 129 P. 467 (1913); Ruthruff v. Ruthruff, 52 Idaho 330, 14 P.2d 958 (1932); Smith v. Smith, 90 Nev. 422, 529 P.2d 209 (1974); State v. Eisler, 270 Wis. 469, 71 N.W.2d 376 (1955).
Where a mother has been guilty of marital misconduct and there is no evidence of demonstrable effect of such conduct upon the child, it does not follow that she is an unfit person to have custody and that an award of custody to her is not in the best interest and welfare of the child. Kester v. Kester, supra; Hershey v. Hershey, supra; Dornbusch v. Dornbusch, 83 S.D. 524, 162 N.W.2d 283 (1968); Septka v. Septka, supra; and Wiesner v. Wiesner, supra.
If the mother’s misconduct is committed in the presence of a child old enough to see and recognize her improprieties as such, then the harmful effect of her bad example upon the child is self-evident.
When the mother, by irresponsible conduct, indicates that her care and custody would be detrimental to the welfare of the child, custody may be awarded to the father. Hines v. Hines, 78 S.D. 464, 104 N.W.2d 375 (1960); Blow v. Lottman, 75 S.D. 127, 59 N.W.2d 825 (1953); Sweeney v. Joneson, 75 S.D. 213, 63 N.W.2d 249 (1954); and Septka v. Septka, supra.
Whether Kody and Kacee Spaulding were of tender years was a fact question for the trial court to determine, and the court did not abuse its discretion in finding that both boys were leaving the age of tender years and were entering upon the stage of their youth when the benefits of association with their father must be considered.3
The judgment of the trial court is affirmed.
DUNN and MORGAN, JJ., concur. HENDERSON, J., concurs specially. WOLLMAN, C. J., dissents.. Miller v. Miller, S.D., 245 N.W.2d 501 (1976); Wiesner v. Wiesner, 80 S.D. 114, 119 N.W.2d 920 (1963); and Howells v. Howells, 79 S.D. 480, 113 N.W.2d 533 (1962).
. We distinguish cases in which custody is initially determined, from those such as Dom-busch v. Dornbusch, 83 S.D, 524, 162 N.W.2d 283 (1968), wherein the party seeking modification of a custody determination has the additional burden of proving that there has been a substantial and material change of circumstances.
. We reconcile this case with Steams v. Stearns, 80 S.D. 443, 126 N.W.2d 124 (1964). In Steams the trial court found from the facts and circumstances that the children, also ages 7 and 8, were of tender years and granted custody to the mother. That finding was within the trial court’s discretion just as a contrary finding was within the trial court’s discretion in this case.