it can't seem to do auto-replies
I beg to differ.
Make sure Canned Responses is activated:
Then you create a filter for emails that are sent to your old email address:
EDIT: Note that this will not work for mails fetched through POP3, only for forwarded messages. If your old account has an auto-forwarding feature, that's the way to go. (The other answer here pointed this out.)
You call the filter, say, Old. Then you select a canned response for that filter. Actually, you don't even need to do that. You can autorespond to these emails just the same just by creating a filter for them.
Look at this screenshot: