Lestat turns Louis in 1791. At that time, Lestat’s been a vampire for 10 years, and he was turned when he was 20 or 21 (it’s UNCLEAR, there’s a debate about this)(even AR doesn’t remember).
So Lestat is 30 or 31 in 1791, which would make his birth year 1760 or ‘61 ;D
I got a similar ask a few months back, it’s got a little more thought on the subject. There’s a timeline in the official Vampire Companion (there is some debate about discrepancies of this book to canon, but this book was intended to be the author’s deciding word on canon facts):
The alcohol swirling in my glass was rather off-putting–I raised it to my lips every few minutes to blend, but Lestat left his drink untouched.
“Louis,” he whispered into my ear. “That one.”
I followed Lestat’s line of vision. He was gorgeous, skin flushed from dancing, and his heart nearly sang, even in the din of the club. He had long, well-cared-for chestnut hair and a lithe frame, perfect for weaving in and out of the gyrating bodies. He wore almost no clothing, choosing instead to parade around in slick, wet-look shorts and a vinyl chest harness. I curled my lip in distaste–part of the fun with Lestat was making him work for his meals.
Lestat wanted to be super duper helpful and make a video to teach all the vampires how do to their highlighting to blend in like a human but then he got carried away. (Quote from [x])
I-want-my-IWTV 