There was also that time that the Feds most advanced warship was protected by a civilian teenager in their new MS. Point being, age has been sort of ambiguous in Gundam. 80 years after Zeta and they're still letting teens as young as 13 be pilots.
Extenuating circumstances
The Federation was getting the snot beaten out of it by people who were willing to drop colonies, shoot nukes, and butcher their neighbors with nerve gas. What's worse, a victory would have meant a liquidation of a large chunk of the population so a madman can construct his ideal world. They were in a tight spot and needed to throw everything they could at them.
The League Militaire was in the same boat. The Zanascare Empire was not known for being merciful. What's more, the League was a bona fide rebel faction fighting without the government's support. Their situation was doubly desperate.
Point being, age has been sort of ambiguous in Gundam. 80 years after Zeta and they're still letting teens as young as 13 be pilots.