I used 18px minimum in Safari for a long time and I'm just 25. If a site breaks, you can just hit cmd+'+' to zoom it up, which will usually fix the layout.

I am now on Chrome which zooms uniformly and when HackerNews comments are just barely readable, the 'reply' links seem to be twice their size, when in Safari they were the same (minimum) size. Neither approach is pretty. :(