While reading Swan's Practical English Usage, I came across this sentence
I don't do much sport now, but I did play football when I was younger.
Why comparative younger? Simply young could have worked there, couldn't it? Or, does it mean that 'younger' means the speaker just left playing sport last year? As it reflects younger as the phase of life earlier than the present year (of his age).
For instance, if the speaker is 45...
I don't do much sport now, but I did play football when I was younger (than this age) -> Up till 44 he played OVER
I don't do much sport now, but I did play football when I was young -> When he was young, say in 20s-30s or whatever.