Joke answer: whomever my wife would let us. Actual answer: I don't know. I'm in my mid 40s. If I had to start over for some reason and was looking for a serious relationship, at least 30s as the minimum. Max would be low 50s, probably, though I've not thought a ton about it. Actualer answer: if I ever lost my wife, I doubt I could/would pursue a serious relationship again or at least for a long enough time to make my answer above probably innacurate.