I really like having a different page for each individual age. This separation distinguishes your site from the countless blog posts that pass on advice "to those in their 20s and 30s" or "to those who are still young". In my eyes, these pieces are often far too general. By this, I mean the following:

Thinking back on my life so far, the years 19, 20, and 22 were quite unique with respect to my then-current circumstances/knowledge/abilities (even though I was in the same undergrad program and had roughly the same job during these years).

If I had the opportunity, I would give 19-year-old me very different advice than 20-year-old me! 19-year-old me really needed it! And don't even get me started on the advice 22-year-old me needed.

Adding some pages that aggregate the advice submitted to similar ages (20s, teens, 18-35, choose your favorite grouping) wouldn't be bad for those who are interested, but I think taking the focus away from individual ages would minimize something quite neat that you've done here.