_Young_ is the time of life before adulthood, synonymous with youth.

When I was _young_. = When I was a youth.  Before my adulthood.  This wouldn't make much sense for a 10 year old to say, but they may.  Also, someone that is 100 may use _young_ to refer to their twenties, even though that is adulthood.

_Younger_ is comparative with the current age.

When I was _younger_. = Ages prior to my current age.  This obviously depends on context.  If you are 100 then younger could be 90, however in the context of playing football that is not as likely.