you won't get a bank account with a tourist visa. And you need a bank account for the retirement extension of stay within Thailand. So, if you want you can do that retirement visa in USA, arrive, open bank account, and do extension process when you're ready to move.
the visa is valid 90 days for your entry to the country. Once you enter your period of stay starts. If you leave before you get your bank account or apply for your "extension based on retirement" you'd probably have to start over. I only know what I've been told as I'm doing the retirement route myself. I already have my bank account.
all are on line. And so long as over 50 you can apply. It's hard to open bank accounts without a visa and you need a Thai bank account for the 12 months extension of stay based on retirement. So get a visa before you arrive, it'll give you time to sort out the bank account.