Sixth forms only get funding for 16-18 year olds (19 if they retake whilst there) so that's their limit, but you can do a-levels at any age. Lots of further education colleges do a-levels- or even evening courses at such colleges, but you could also take them as a private candidate taking them online or self-taught.
If there's a particular course you want to do with those a-levels, you could do an access to higher education course, or try a uni foundation course or apprenticeship to get there.