I was working on mobilizing a site recently and was asked about font sizes. Also was talking to someone who was ordering tablets for staff because everyone could read them because of screen size.
Traditionally you could put increase font size buttons on a site then store it in the session. I’ve done this in the past on public sector sites that were insistent on passing WCAG and WAI guidelines. While that is well intentioned I’m not sure its a great way of sorting out the problem or even passing the intention of the guidelines.
Users don’t want to be pressing an increase font size button on each site they visit that may or may not be there and is going to be in a different position on each site. It seems better for users to control font size in their browser either permanently or with the browser on an odd page. That way it is stored and more permanent.
Their eyesight isn’t going to change from website to website, barring laser eye surgery in your lunchtime.

That said its worth bearing in mind when designing sites everyone benefits from clear text and a site layout that stays readable even with the text resized according to browser preferences. Not only the middle aged or people with reduced vision but the tired eyed genius teenage programmer of stereo type.

The BBC offers good guides on text resizing if your trying to setup a browser for someone else.