Concerning Age in Relation to Appearance

.: The Laws and Customs Among the Eldar :.

"The Eldar grew in bodily form slower than Men, but in mind more swiftly. They learned to speak before they were one year old; and in the same time they learned to walk and to dance, for their wills came soon to the mastery of their bodies. Nonetheless there was less difference between the two Kindreds, Elves and Men, in early youth; and a man who watched elf-children at play might well have believed that they were the children of Men, of some fair and happy people. For in their early days elf-children delighted still in the world about them, and the fire of their spirit had not consumed them, and the burden of memory was still light upon them.

"This same watcher might indeed have wondered at the small limbs and stature of these children, judging their age by their skill in words and grace in motion. For at the end of the third year mortal children began to outstrip the Elves, hastening on to a full stature while the Elves lingered in the first spring of childhood. Children of Men might reach their full height while Eldar of the same age were still in body like to mortals of no more than seven years. Not until the fiftieth year did the Eldar attain the stature and shape in which their lives would afterwards endure, and for some a hundred years would pass before they were full-grown."
   - Morgoth's Ring, 'The Laws and Customs Among the Eldar'

.: My Guesstimate :.

   Most human males reach physical maturity at 18 years of age, but some may continue growing into their 20s; whereas elves are full-grown by the age of 50, with some taking until they are 100 years old to mature.
   At 18 - 20 years old, elf-children resemble 7-year-old humans, so I'd guess that when they're 50, Elves probably look like a young adult, and therefore like tweens when they're roughly 25 - 35, and like teenagers from 35 - 45 or so.
   If Feveren, at 51 years of age, looks young enough to be mistaken for an elven adolescent by humans, hobbits, dwarves, and even other elves (though High Elves are, most likely, more perceptive), I imagine he probably seems to be in his mid teens, i.e. about 40 in "elf-years".

*      *      *

Top: Feveren at 30 years of age (III 2996)
Middle: Feveren at 51 years of age (III 3017)
Bottom: Feveren's stature and girth (III 3017)
- Courtesy of Perchance AI Character Generator