What you mean by marrying?
Do you have a charge controller?
Do the older batteries show a reasonable voltage when fully charged and when part depleted?
A battery life is determined for example by loading it with a known current and measuring how long it can provide current before its voltage drops to 90% of it's rated voltage.
If it can provide a current of 1A for 1 hour then it could be said to have a capacity of 1AH.
The 90% figure is not set in stone, you could say 85% or 95%, but as long as you are using the same benchmark for each battery you test then you have useful data for comparisons of "life"