Among my very small list of skills, I happen to be rather skilled at guessing babies and toddlers ages. Maybe because I did lots of child care? I don’t know...
But they look to be about a year apart; not just the sizes, but the maturity in the faces. The lighter boy may tall for his age, and the darker boy likely is not, so the size differential in height is not so telling. The younger boy has a much smaller head and face and more of a “baby” look to him, the darker boy looks much more mature, does not have the baby belly, has more muscular development.
I’d like to say 2 to 2 1/2 for the lighter boy and almost 4 for the older boy. If we ever find out then I’ll know how close I am.
your observation indicates that the family group photograph was taken not very long after the 'two little boys' - the dark boy seems to be about 4 going on 5, and Mark - how old would you estimate him to be? I am under the impression that photograph dates from around late 1966.