For a cursory understanding of whether you will stay together is this (note that I did not say "you all will be successful". Because startups rarely do what they set out to do; they change a lot during initial phase):
0. How long have the two co-founders been together? What is the chemistry between them.
1. How resourceful are they? Will they be able to make things work when things start to looking gloomy? Gauge their energy level and enthusiasm in general.
2. The project they are working on, how far are they in the project? How did they achieve that progress?
Get answers to these questions and you will be in a better position to make a decision.
Founder age does not (and should not) matter. Being junior has its own set of advantages.
And even if they don't have prior startup experience their resourcefulness should cover that up. Even experienced founders fail.