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.
Best Answer
The sentence
is the correct sentence and can also be expressed as