IRL, humans reach their maximum height around age 15-17.
If you are younger than that, you'll probably still gain inches.
However, the range of human heights means that some 13-year-olds (the ones who will be over 6 ft when they reach their maximum height) are going to be much taller than some 17-year-olds who have already reached their maximum height.
I'd use the Young Characters rules in Ultimate Campaign as a benchmark here. Those rules allow characters as young as half the base adult starting age for their race, with no change in size category. So for a human, for example, adulthood is 15 years, and starting age is 8, plus either 1d6 or 2d3 (depending on which NPC class they start in).
It's kind of a weird question considering how people grow at different rates even just among humans. I know a 6ft 10 year old and under 5 foot 20 year olds.
The general size benchmarks are that every size category is twice as large as the previous one, as conditions that increase your size also double your height, and conditions that decrease your size halve your weight.
The general benchmarks for Medium-sized creatures is 4~8 ft. Some of this can be seen due to Dwarves (height range of 3' 9" to 4' 5", averaging as just barely Medium) and Gnomes (height range of 3' 0" to 3' 8", averaging still below this four foot threshold).
Cross-referencing this with the average size of children... and your average 8-year-old is already tall enough to qualify as a Medium-sized creature. By the time they're 10, they're taller than Dwarves.
As Youthful Appearance can't change size categories, there would appear to be a limit on just how youthful they can be.