Actually, we made 3 bookings for 5 of us because as EBG if you buy Go you get 3 checked bags and that's what we needed. So kid #1 booked go and the rest had go light. At check-in (bag drop) at MMX, the check-in agent was checking the DOB of kid #1 and said she had to check that kid #1 was indeed less than 26 years old (she was reading the DOB of kid #1). Then at the gate they called kid #2 and #3 to check their passport (and they were told that they had youth ticket and they had to verify their age).

if they want to check picture ID they would have to check everyone's picture ID, not only the people travel on youth tickets.

It would be very easy for everyone to cheat if you fly within Schengen because there is no ID check.