I haven’t heard of anyone who is older sitting the eleven plus. I’ve heard of it the other way round and DD’s grammar (London Borough of Bromley so not Kent, but DD also sat Kent and Bexley tests a few years ago) admissions for her school state that only those below a certain age can sit the test, of below the normal admission age they can try for the test if they are in Y6, I.e outside of their age cohort but it is their usual school year for them. They can also only do the test once.
Is there anything in the admissions materials for the schools you might be interested in (appreciate that is a long time away!) that clarifies this for you? I note that it says on the Kent test registration page to contact the admissions team if your child is outside of the birth dates for the test.
https://www.kent.gov.uk/education-and-c ... test#tab-1