I get why you might be frustrated, I’m not sure how much VC you’ve read, I hope this answer doesn’t spoil you if you don’t want to be spoiled but here we go… 

Lestat was born in 1760. Not sure where you got 1780… I believe you are forgetting the almost 10 yrs Lestat and Gabrielle ~wandered the Devil’s Road~ together, which was from 1780-89. 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):

Here’s a snippet of the time line, sorry for the warping, but it’s a thick paperback book and I didn’t wanna injure the binding!

Worth noting also: AR underscores that Lestat was 20 when he was turned.* 

So by 1791, Lestat is actually like 31 mortal years old, and Louis is in fact younger than he is, at 25 (why they made him 24 in the movie is beyond me bc it’s only a year different? IDK).

*Re: Lestat being turned at 20 or 21, that’s a topic that’s been debated for aaaaages, too, since he says in his book, “In the winter of my twenty-first year, I went out alone on horseback to kill a pack of wolves” and more confusingly, in the timeline above, AR indicates that Lestat is 19 when he goes to Paris w/ Nicolas so I am confusion, but taking Lestat’s line from TVL alone, that it’s his “twenty-first year”:

• a baby’s “first year” is actually the day it’s born through it’s one-year birthday; 
• therefore, Lestat’s “twenty-first year” could be his year from his twentieth-year birthday through his twenty-first year birthday. 

(Not that his birthday was celebrated in his house anyway, so it’s really just about whether he gets carded at bars, currently, and he totally does.)

I headcanon that:

• Lestat kills the wolves in the winter, sometime after his twentieth birthday, 11/7/1779, 
• and then goes to Paris w/ Nicki that spring (5/1/1780 maybe).
• Lestat and Nicki are working at the theatre for only a few months and then Lestat gets his big break in August: “It came in late August at last.” 
• Lestat only gets to perform for a few months, at least through October (”But in the month of October when Paris was already freezing,…”).
• and then Magnus turns Lestat sometime close to, or before, his actual twenty-first birthday 11/7/1780. Maybe in October or that first week of November.