They were keen to bring it down to 24, but given current shortages they've gone a bit quiet on it at the moment.
I used inhalers as a kid up to the age of 13 and joined the RN (Observer route) at the age of 23. I was honest with them from the start about it and it proved to be no problem. Don't know if there's a length of time required where you haven't been on medication though. If there's any doubt they'll send you to be tested by a doc.
Hope this helps.