Average fleet age is only 2.9 years older at United. The average age will probably the same in 2 years when all the 747's are parked. United and American are getting lots of new ac over the next few years. This is not a reason to pick one vs. the other.

United pays more now, American will pay more in 2 years, United more 2 years after that. Pattern bargaining. Again, not a good reason to choose one over the other.

I wouldn't pick a wholly owned like Envoy to get to American. Probably the slowest way. Skywest guys have been going to American with half the hours Envoy pilots have.

Go to the regional that will give you the best quality of life, then go to the first major that calls. After a year at the first major, most likely it will no longer be worth it to leave for another major for quality of life reasons. You would be giving up a lot of seniority to do it. It could make sense though to be based where you live.