Twenty-five...
By that age we had finished our undergrad, were established in our careers, prefrontal cortex was fully developed and mature...
At that point we were able to accurately assess the risk and reward, as well as the return on investment from the partnership.
We discussed, agreed, and were compatible with regard to:
1) Children (no thank you),
2) Money (yes please)
3) Religion
4) Each other's families
5) Life Goals and aspirations
6) Political Leanings
Any younger than twenty-five and I am not sure we would have fully appreciated the gravity of the decision.
We have been married twenty three years, and it was the best decision I ever made, not to mention my best investment!