Why Do We Age?

I've read that the purpose of life is to reproduce and continue our species. Darwin says that we have evolved to that purpose. So, if that's all true, why do we age?

When we meet an attractive person, all our instincts say: "reproduce". And, especially for men, it says: "do it now, before the opportunity is lost". But, as we get older, we age and our attractiveness to the opposite sex wanes.

The logical reason for this is that the likelihood of having healthy offspring is enhanced if we mate with someone who is physically fit. One major indicator of health is that you look attractive.

But, wouldn't it be better for the continuation of the species if we didn't age at all. The mating would continue for that much longer with all of these attractive people continuing to meet each other. Then, one day, when it was no longer advantageous to the species, we could just expire.

I'm just asking.