It really depends on what you mean by “learning violin.” It varies based on what music you’re interested in playing.

Probably the best thing to know, regardless of what type of music you want to learn, is that the violin is one of the more difficult instruments to master, but for millions of people it is enthralling, whether playing or listening. It’s a lot easier to sound good on the guitar or piano or other “fixed pitch” instruments even if you’re not very good, as if the instrument is tuned correctly it can sound ok. The violin doesn’t have guitar frets, and it’s not as simple as striking a piano key – you have to place fingers very precisely in order to play in tune (though the guitar and piano have techniques that the violin doesn’t which can make them practically as difficult in their own way).

Folk, country, bluegrass, Irish, Gypsy, etc., really anything other than classical, are much easier. Jazz sits between those first types and classical, as far as difficulty goes. Age 21 is considered late to get started for any type of music if you’re looking at playing professionally, but has been done very successfully based on dedication and practice.

For classical, you can have some fun, and I’d always recommend it, but realize that to get professional quality normally takes at about 15–20 years of diligent study, and requires a pro teacher. All the best classical violinists I know of started age 3–8.