I can't find my general class license. I want to apply for a vanity call sign (my call sign when I was 16 years old) but I don't know the expiration date of my current license. Is there some way to find the expiration date online?
Go to the FCC Universal Licensing System (ULS) web site and search for your license. You can search by call sign, name (in lastname, firstname initial order), or address (in the advanced search form). It will show you the expiration date and current status.
If your license has already expired, you can submit a renewal up to two years afterward. From a review of the form, it looks like you can combine a renewal with a vanity call sign application, but I'm not sure.
If two years has passed since the expiration of your license, you will have to take the Technician exam over again, but you can get exam credit for your previous General license.
Note that there is a very good chance that "your call sign when you were 16" has already been claimed by someone else if it is shorter than modern sequential call signs; short vanity call signs are in high demand. The first thing you should do is search for that call sign in the ULS, even though it's not the one you currently hold, to check if it is available.
For licenses in the USA you can go to this site FCC ULS
Scroll down the page to the entry for entering your call sign, then scroll to the bottom and click search. It should find your license no matter if it is current or already expired. You can click the call sign it finds to see all of the detail including your FRN number you can use to attempt to renew the license. The license should be good for ten years and you have a two year grace period after that for renewal. Renewal can be done online and costs nothing.