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 boys reach physical maturity at about 18 years of age, but some may continue growing into their 20s, while girls generally mature between 16 and 18. However, Elves are commonly full-grown at the age of 50, with some taking until they are 100 years old to fully develop.
   At 18 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 52 years of age in III 3018, 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 appears to be in his mid teens, i.e. in his mid 40s in "elf-years".

Stature and girth (50 years of age)

*      *      *

Growing up:

Elf children resemble those of Men until 3 years of age, after which their rate of growth slows. Taking the span of years for Men between ages 3 and 18 (15 years) and applying it to elf-children, who appear to only age 4 years over the same period, means each apparent year of their growth is actually 3.75 years. So an Elven child that looks like a 4 year old child of Men is actually 6.75 years old (3 + 3.75 years).

3 yr old Elf still resembles a 3 yr old Man
6.75 yr old Elf resembles a 4 yr old Man
10.5 yr old Elf resembles a 5 yr old Man
14.25 yr old Elf resembles a 6 yr old Man
18 yr old Elf resembles a 7 yr old Man

After which it must be assumed that the average Elf's growth rate increases, otherwise they would only look 11 at 33, 16 at 50, and 18 at 60 years of age; it would take until they're 104.25 years old to appear 30 (as Feveren seems destined to do!)

Feveren at 6 (III 2972), 12 (III 2978), 18 (III 2984), 33 (III 2999), and 52 (III 3018) years of age.

Stature:

Going by the average height of a human boy, at 3 years of age Feveren would stand at 3'2" (95 cm), and:
3'8" (110 cm) at age 12
4'0" (121 cm) at age 18
4'9" (143 cm) at age 33
5'8" (169 cm) at age 52

- "Concept art" and "photo"courtesy of Perchance AI Character Generator

Please note that this is all based on Laws and Customs among the Eldar that JRRT wrote in the late 1950s and therefore doesn't include his subsequent pondering on the subject as published in The Nature of Middle-earth.