Waaaay back there was a cap of age 36, but that was relaxed for a variety of reasons including conversion courses for Flight Service Officers and overseas ATC recruitment.
Older trainees (i.e. around 30+) with life experience, having had various jobs & tertiary study and often married tended to do just fine. The fact that some were married was an additional motivation to knuckle down and study, given their family responsibilities and probably not another job to fall back into
Performance in the assessment test phases are a key factor - recruitment process steps 2 & 3 here:
Trainee air traffic controllers