why is the gi generation so big

what i mean by this is why is this generations age range so big i mean it begins in 1901 and ends in 1927 and i’m just curious as to why the start date is early like that? just bc of the fact that the older ppl of the gi generation basically didn’t fight in either ww1 as they were too young for it but also weren’t really allowed entry for ww2 as they were considered to be too old. so thats why i was wondering why the age is so big for this as this generation is heavy on ww2 for obvious reasons and some of the older ppl of the gi generation couldn’t acc serve in ww2 and i understand that it was a joint effort of ppl of all ages but it still doesn’t make much sense to me at least