One way to hold this word: time has passed, but it left no marks you can count. Look at the thing and there is nothing to measure — no lines that say sixty, no style that says the nineteen-eighties, no date at all.
The form itself is age + -less, the same -less as in endless or useless: without age.
字形上就是 age 加上 -less，跟 endless、useless 裡的 -less 一樣，意思是「沒有」。所以字面上可以想成「沒有年紀」，而兩個意思就是「沒有年紀」的兩種讀法。
Wiktionary entry for ageless (kaikki:3f6f69a190036ae4:928565039:15352); the packet's etymology tree traces the age element back through Latin aetatem.
Two branches
看不出年紀
這一支讀的是「沒有年紀」的外表層：年數確實累積了，只是看不出來。
adjective · B2
Looking the same as the years pass, so you cannot tell how old it is — used about a person whose face never seems to get older, and about a style or design that never starts to look out of date.
不顯老；不過時
久到量不出來
同樣是「沒有刻度」，但這一支量的是真實時間，不是外表：往後看不到起點，往前看不到盡頭。
adjective · C1
Having lasted for so long that no starting point can be put on it, and looking as if it will go on without end — used about mountains, old buildings, a feeling, or a kind of charm.
亙古不變的