You are quite right, my argument didn't hold water. The level advancement in D&D doesn't really make much sense when you look at the time it takes to get from level 1 to 20. Depending on the campaign, it can easily be done in something like 2-3 months game time, at least in 4e: 4 encounters/day, 10/encounters a level = 50 days to level 20, 75 days to level 30.I'm sure you do look at it that way, but it's not RAI or RAW, especially as PCs progress from level 1 to level 3 very rapidly, RAW, in 5E. Likely in adventures that take a period of days. Level 1 to 2 apparently takes about 1/4 or 1/3rd as long as the rest of the levels.
So now you have PCs are who are 16/17-year-old runaways who level 3...
A more sensible split, given humans are a long way from physically or mentally fully developed at 16/17 (human males cannot remotely approach their peak strength or endurance at that age, for example, and even fantasy novels, they're not going to), would be to use a different stat-generation mechanism for teenagers (or modifiers), than from adults, if that's the differentiation you want.
A better to look at it to make sense is that before you have been in combat (and gained xp/levels), you just can't translate what you know (archery for instance) into good attacks.