The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
125. Adjectives in -er form the Superlative by adding -rimus to the nominative. The comparative is
- ácer, keen; ácrior,
- miser, wretched; miserior, miserrimus.
a. So vetus (gen. veteris) has superlative veterrimus, from the old form veter; and mátúrus, besides its regular
superlative (mátúrissimus), has a rare form
For the comparative of vetus,
vetustior (from vetustus)