I like the aging idea, but why would you ever go past 4 age counters? Obviously not efficient enough against enemy creatures.
Instead, it should affect all creatures in play (including enemy). The aging cycle should be much shorter, and less boosting, like:
(all end-of-turn effects)
1 age counter: +0/+1
2 age counters: +1/+0, gain mitosis
3 age counters: nothing
4 age counters: -1/-1, lose mitosis
5 age counters: die.