And if you consider the anime's thought of the pokemon being more intelligent than the humans, then it makes even more sense. Pokemon's humans' brains might be more beefier than ours and can handle "adult" things at a lower age to insure survival (even if they're lower in intelligence). We don't know anything about the physiology or the society of the humanoid species in Pokemon's universe, so we can't say it doesn't make sense just because it doesn't fit our way. Lower chances of survival also helps make sense of lower age of marriage.
Just because they look, talk, and act like us doesn't mean they are us. Age is relative, ours only hold on basis with our species; you can't use our measurement of "adulthood" to other universes (especially when it barely works for us anyways). Physical age hold not that much influence on maturity levels/brain age.