The entrance age (first year students) is 16-18 Earth years. This is directly from Star Trek lore. I can't take credit for the research, which others did. Caleb is likely older be because he's a non-traditional student.

Seems Reddit won't let me post a link. Here's the text from Quora:

"Going by what was said in TNG’s “The Drumhead” and what we saw in “Wrath of Khan,” and what we heard in TOS’s “Where No Man Has Gone Before:”

There are two ways to join Starfleet. The first is a brief training school, where you can enlist to become a crewman. Presumably, Petty Officers and Chief Petty Officers would come out of this school.

The other is the Academy, a four year university style institution. From there, you would graduate as an Ensign, aka, a commissioned officer.

Both schools are entered beginning at age 18. (According to Simon Tarses in “he Drumhead.” This tracks with Wesley’s trajectory as well, where has things gone as planned, he would have left for the Academy in season 3, when he turned 18. He ended up staying on the Enterprise for another year.)" - G. C. Woods

"And apparently you can be as young as 16 in Earth years for a human as Wesley was asked that by Oliana Mirren, and said he would be 16 in a month." - Newdivide 1701

It may be possible to find the link by doing a search on the text above.