My last fortress was 42% children and babies though lol. I looked up the US resident population by sex and age in the year 2000, and the percentage of humans aged less than 20 in the US was
(19176 + 20550 + 20528 + 20219) / 281425.0
which is 21% 28.5%.
So even counting every human under 20 as a child, that dwarf fortress had, proportionally, twice the way more children.
Pre-modern population distribution was *significantly* different than what we are accustomed to today. Remember that DF is set pre-1400. A quick check shows that in the 1910 census, which is still far more modern than medieval, the US population had 32.1% children, defined as age 14 or less; and this doesn't count the still significant mortality rate of the very young. If I' reading the 1890 tables right, it was 35.5%. Earlier censuses had less specific questions, but the median age for the entire US (all classes/races/sexes) in 1820 was 16.7 years old; the earliest US census data (white males only for 1790) show a median age of 15.9 years. (By comparison, the 2010 US median age across all groups was 37.2; a mere 220 years back drops the median age by well over half, and DF is intended to be set the equivalent of over 600 years back.)
Picking a semi-random well documented case,
King Edward I (1239-1307)... had at least 14 and perhaps 16 children by his first wife, of which only 6 survived to adulthood. When his first wife died and he took a second, he had 3 more total, 2 of which made it to adulthood. Note that this was a family that was, in general, well-supplied with all its physical needs; yet less than half of them made it to adulthood.
Population growth requires, in the most general terms, that a couple have an average of more than two children that survive to adulthood and bear children of their own. Given that not all adults are able to have children (become priests, die in accident or combat before having children, many other reasons), you need noticeably more than 2.0 surviving adult children per couple; given child mortality rates well over 50% in many cases, a couple having 5-6 kids was quite common, and 8+ kids wasn't significantly unusual in many places.
tl;dr: DF is attempting to simulate a pre-1400 medieval population. A substantial fraction of your population *should* be children for a vital and thriving fort; if you don't like that, feel free to mod it to something more to your liking, but be aware that you're getting into the category of mods that make the game easier by making it less realistic to your personal taste.