Can I still sign into an old 360 from 13 years ago with no info besides Username?

Hello, first it's a pleasure to be able to help you today.

My name is welcome to the community.

Hello GDubach, unfortunately to have access to this account that used in 360 you would have to have that yahoo email that had created.

Even if you only know gamertag you can't recover an account with Gamertag alone.

meh...kinda rather it not be backwards compatible anyway.....keeps the comp cycles free for the current (next) gen. gaming....they can focus on the new and not worry about the old.....besides they know there will still be a market for 360 stuff more money
to be made if people still have to replace their 360 from time to time....I wonder if they will follow sony's example and sell 360 stuff years after the 720 is released (if I'm not mistaken, you can still get new PS2's and Ps2 games)

Afraid not.The locking of modded achievements/gamerscore is permanent.

If you want to unlock any reset achievements legit then you'd need to start a new account/profile.

How old is your account? EA games require you to be at least 13 years of age to play them online, no matter what the settings on your Xbox are besides that. So, if the age you set up on your Xbox is below 13 years of age, then you are not allowed to play
EA games online until you reach that age.

There was a video on it, the guy logged into his account using his old Xbox 360, worked fine but he had to pay some money so he got the 15 letter long name he wanted. I tried it and you just make a new account and login with your new xbox and just login with the Xbox 360 to change it to 15 letters instead of the 12 letter limit that newer gen consoles have. This is great, but I still need access to screennames used up like 15 years ago where the players have abandoned them like 13 years ago.