There is a phase in pretty much everybody's life - which in men tends to manifest in the form of drawing penises pretty much anywhere - usually in one's teen years when that kind of humour is widespread.

Then most people get too full of themselves and concerned with appearances and lose that kind of humour (some will even act shocked when they see it).

So I read the reference to the age as just indicating one of those rare people whose humour hasn't, as they aged, been crushed by a need to be seen as a serious person, which I personally find are the best kind of people around at many levels, not just the humour itself but because that stuff tends to be correlated with self-confidence and being an easy and fun person to be around.