How old was Jason when he died, at what physical age was he resurrected, and is it true that the lazuris pit gave him extra benefits?
This is a really good question. Let's start with the basics, and lets assume that 'today' os always when the comic it hapened in came out, unless stated otherwise. Most sources agree Jason is 12 when he becomes Robin, at least for New-Earth/Earth-One. New 52, in an attempt to put all of Batman into 5 years, made him 15, but Rebirth/Infinite Frontier stretched that out, so we can ignore the 15 age.
In comics, Jason was Robin for 3-4 years before democracy kicked him. If we used that, that would make Jason 15-17 years old when he died, most likely leaning late 16, maybe 17, as he can mostly get a flight on his own. Jason is dead for half a year or so until he gets brought back to life, physically and mentally still 16-17, even after his 18th birthday is celebrated by Bruce memorializing his death.
Edit: Due to a comment left by @liedownquisition (thanks BTW, really) I was reminded of the alternative (and probably much more likely number). They are very correct when they say the certificate that only shows up twice is only a little canon, the price we pay for comic books I reckon. There is just as much evidence for the time between Jason's sidekicking and death only being 1-2 years, if even. This would place Jason dying anywhere from still being 12 (depending on if it even got to a whole year) to 14 years old. If you wanted to take an average of all these possible ages, it's 14. Apologies for leading you all astray. (Still fucked up a kid that young died though).
In terms of the Lazarus pit giving him powers, yes. We don't know the exact extent of those powers, but we know the Lazarus Pit has given him acess to magical abilities. For now, all we know is he can remove power and summon the All-Blades.
I may have missed some info, but as far as I'm aware, this is the biblically accurate BatFam Jason answer.
