I always assumed Gil & Agatha were both 18 but I can’t find it actually stated explicitly in the comic or the Secret Blueprints (though those say he’s only been dragged around the Empire getting tested for a month, he pretty much just left Paris, I’d thought it had been a little longer than that) but it seems a lot of the fandom is saying Gil’s a few years older than Agatha. Is that from the novelizations or something?
It’s stated in the novels that Gil turns 22 while Agatha is with the circus (giving him a spring birthday and having him be 21 during the Castle Wulfenbach arc) It’s also implied that Agatha has only recently turned 18 that winter (as the comic starts in March). In a reddit interview from a while ago someone asked for Zeetha and Gil’s ages and was given 21/22 as well. (Which means they are almost 25 (Will be once spring comes) post timeskip since they lived through it). Tarvek’s age is never stated though as he seems to be around Gil’s age in the backstory on Castle Wulfenbach. It’s pretty simple to think that happened around when Gil was eight (or late 7) since Zoing was his breakthrough project and breaking through because he lost his only friend makes some sense there.
My understanding is that a lot of the timeline weirdness is because originally the Other war lasted only a year but was then retconned into three years, forcing Gil’s age to increase. The retcon happened pretty early on I believe but it dose mess with a little bit of the early stuff.