Voadam
Legend
The two youngest I've seen were in my current 5e Iron Gods high tech spaceship crashed in D&D game.The youngest character I've ever seen played was 11 years old. Fully adult for its species (almost-full-blooded Orc), however, which had a normal maximum lifespan about half that of a Human. Our tables allow you to start as young as about Human-equivalent 17, and yes our system still has age affecting stats (which in many ways is a very sensible rule, if imperfectly implemented in 1e).
Our roll-up tables don't allow a character to start beyond about (Human-equivalent) 40-ish years old, but get unlucky against a Ghost and you can very soon be playing someone in their 80s.
One was two years old. He was a Werewolf the Apocalypse lupus werewolf concept though (reskinned human druid), so the two-year old wolf with tons of lore instincts and racial memories looked like a teenager when in human form.
The youngest though was the robot (reskinned warforged artificer) who got activated in session 1 by a technomancer using cobbled together parts from multiple robots. So literally started off at newborn age.