How old was Arthur Dayne?

[Vaedys Targaryen]

I am wondering how old was Arthur Dayne and when did he join the Kingsguard?

Rhaegar Targaryen was born in 259 and died in 283, which means he died when he was around 23-24 years old, making him die relatively young. Since Arthur and Rhaegar were best friends, I suppose it could imply that they had had a childhood together and were thusly around the same age.

But we know that Jaime was the youngest Kingsguard so far in history and Arthur was the one who knighted Jaime when he was only 15 and he was already in the Kingsguard at that point.

So how old was he? And when did he join the Kingsguard?

[Adam Yozza]

I always pictured him as fairly young,in his mid twenties and a few years older than Rhaegar. Arthur was in the Kingsguard when fighting the Kingswood Brotherhood, and that fight ended in 281 so he'd joined prior to that. The wiki; though I'm not sure how accurate that is; says that he was born in or before 260. So the lowest possible age for him would be 23 at his death, though I'd probably imagine him being a few years older, maybe 26 or 27.

[Farstrider]

Mid to late twenties.  Old enough to have honed his skills and earn his reputation. 

[Laughing Storm Reborn]

being rhaegar's friend doesn´t mean he was the same age... if i had to bet, based on jaime being the youngest kg ever (so dayne was not apointed at 15/16, maybe 17/18 minimum) and arthur having a solid respected reputation as early as the KB outlaws skirmish, he had to be 28/29 when he was killed

[Hangover of the Morning]

Arthur's age is never specified so George can reveal it to be whatever he wants but count me among the people, who thinks that Arthur was on the younger side: Rhaegar's age give or take few years. Simply a hunch and combination of the points below:

• It's unlikely to befriend someone more than a couple years older or younger when you are a child or a teen and since Arthur was not only Rhaegar's dearest but also oldest friend, it's safe to assume that their friendship goes way back (maybe they squired together in their youth or maybe Arthur was "fostered" for a time in KL as a kid since Rhaegar was de facto an only child and it's customary to bring other noble boys to keep the prince company)
• The age of Arthur's siblings: the older brother had his first child two years after Arthur's and Ashara's deaths, while at the same time the youngest Dayne is but a wee kid when they died. Unless they had different mothers, it's unlikely that there was more than a 25 year gap between Edric's dad and Allyria. Arthur has to fall somewhere in between. 
• Ashara and Arthur were very close by all accounts and if there was a significant age gap between them, they'd barely known each other. Ashara couldn't have been much older than Ned and Robert as she's repeatedly described as a "young maid" and "new to court" around the time of Harrenhal Tourney. Edric left Starfall to become page and later squire at the age of 8. Jaime left home when he was 11. Arthur would presumably not be an exception in this case and would have left home around that age as well. He and his sister could have bonded during their time at court but we don't know if they were even in the same place. Ashara, as one of Elia's ladies in waiting was probably on Dragonstone rather than King's Landing but we don't know about Arthur's whereabouts.
• I always took for granted that Arthur joined KG at a young age (around the time he turned adulthood: 17-20) but not absurdly young like Jaime (otherwise it'd be noted). Similarly, I got the impression that Prince Lewyn joined later in his life (e.g in his forties). No textual evidence, just feels right regarding what we know about their personalities. Arthur probably joined after Duskendale (277 A.D.), otherwise he'd be mentioned doing "something" at that time. So if Arthur replaced Gwayne Gaunt (who died at Duskendale) and was 20 at that time, he'd be two years older than Rhaegar. He could have also been younger and/or joined a year or two later. Either way, he'd have 5-6 in the KG plus few years as a Sword of the Morning and a celebrated tourney knight prior to that, which should be plenty of time in GRRM-verse to achieve eternal fame. 
• Of all the celebrated knights in Aerys' Kingsguard, teenage Jaime identifies the most with Arthur. Could be because Arthur knighted him or because he was the best swordsman or because Jaime liked his personality (or looks) but could also be because he could related to him the most due to their closeness in age. The White Bull and Barry the Bold were famous throughout the realm and had much more victories under their belts compare to Dayne but no sane teenager wants to emulate someone 3-4 times their age. A guy who is already famous but only few years older than Jaime sounds like a much more reasonable role model. We don't know the age of Whent, Martell or Darry but they all had siblings in their forties so would likely be in early thirties at least.
• GRRM likes his heroes absurdly young when they achieve their fame (Daeron I, Jon, Robb, Jaime, Ned and Robert during RR) so why should a prodigy like Arthur be any different?

[BRANDON GREYSTARK]

The same age as Brandon Stark .

[Hangover of the Morning]

22 hours ago, BRANDON GREYSTARK said:

The same age as Brandon Stark .

What do you base it on?