The Doctor likes to think he improves with age. When he first started flitting around the universe in his purloined TARDIS, he’d been hardly more than a callow youth, despite resembling an elderly man with a touch of the mad scientist.
Some of his regenerations look younger than others, making it seem that he’s ageing backwards, but no matter how youthful he looks, each new body still contains all the accumulated knowledge and experience of his former selves, even if it’s not always readily accessible.
Age doesn’t necessarily equal maturity though. Youth is for those old enough to enjoy it.