(a little baby Ravion, probably reading a fairy tale about a brave warrior saving a beautiful prince from an evil queen)
Short answer - we don't know.
Longer answer - we still don't know (unless it's actually mentioned in the game and I just missed it), but there are some clues we can interpret. I'm not sure these clues are intentional and are supposed to hint at his real age, so take my speculations more as a headcanon. I don't think they'll ever give us a canonic age for him anyway.
0. To set a broader time frame: the Essail first escaped into the caves about 2000 years ago, led by the queen and sage Tasso. And Sal'thorin was built 300 years ago (according to Zanie). We also know that Velara and Thador are older than Ravion (they are mentioned in the queen's will she wrote before giving birth to him.) So no matter how supernaturally old he is, they are older. We don't know how long an average Essail lives, but I think Thador mentioned that they pay with their longevity for drinking the Divine Dew. So I presume an average human lifespan for a normal Essail.
1. We know that he spent a long time being kept mentally and physically a child.
His lore describes the document he read to find out about his birth as "ancient." For me ancient is not decades, it's rather centuries or even millennia. So he was a child for a really long time.
1.1. The queen first felt the need to cleanse her corruption after Sage Tasso died, and she first started experimenting with dragon eggs. So it probably happened very early on in the Essatil history. So is it possible that he's nearly 2000 years old and he spent most of it as a child?
1.2. I feel that having spent this long as a child left a mark on him. It might explain certain immaturity he possesses, despite being cunningly smart and capable when he wants to. I mean, he thought that making Indris drunk and having him hug some cats was the funniest shit ever. But also he seems kind of apprehensive of kids.
2. Then he "lost" the brooch that repressed his development and, according to him, aged up 10 years in only one year. (Probably similir to Kulu, who's a 5 years old child who looks like a teen.) Then he proceeded to age up normally, I presume. I don't think he's been an adult for an unnaturally long time.
2.1. There is a young-looking servant NPC in the palace who talks about how nobody was allowed to fish in the ponds inside the palace, except for Ravion. I presume he meant child Ravion. We know from his trailer that he's still fond of fishing, but I doubt he'd visit the palace as an adult just to fish there. So, since that NPC remembers child Ravion fishing, then he probably hasn't been an adult for very long.
2.2. He said himself that as soon as he grew up and left the palace, he immediately came into power, becoming equal to Velara and Thador.
I don't see why he would wait to implement his "let's have handsome commoners in the guard" program. So by the time he first met Indris, he was probably fresh out of the palace.
3. On the other hand, Xera (about the same age as Indris) called him something like "an old fox," implying that he's older than her. I mean, technically he is extremely old. And I don't think the queen bothered to hide his existence from the world, so people probably know that he was born at least centuries ago.
3.1. In his tavern dialogue with Thoran, he did say he's much older than he looks and compared himself to Thoran's brother in terms of age. But who knows how old this is?
Bonus: How old is Feli?
He's referred to as a child. But.
It's weird for a grown-up such as Indris to be best friends with a child. We know that he was an adult when he became a guard and he met Feli on his patrols. So if Feli still was a child by the time of his murder, he was an even younger child when they first met.
(But then again, when he met Damian, who's a child and has been mentioned to be about the same age as Feli, he immediately befriended him too. So, I don't know, maybe Indris missed his true calling as a teacher)
There was a line from Indris, something about how he always treated Feli like a child. Why would you specify that you treat a child like a child?
I think Velara referred to Feli as a child, but she also called Bonny and Ravion children, so who knows.
But all things considered, I still think he's supposed to be a child.
