I really enjoyed Gary Gygax's Gord the Rogue books. I don't love them as much now as I did when I was younger, but I do still like them. Their contents really stick with me, for good (excellent fight scenes, for one, and cool monsters and vile bad guys) and for bad (he's kind of a sexist jerk a lot of the time, honestly, which bothers me more and more over the years.)
One thing I especially enjoyed was the writeup of stats for Gord, going from these as a kid:
. . . to these as an adult.
We never did the aging rules in the AD&D DMG. I'm not sure we really cared about age until my last campaign. We used aging like the we did the rules about weapon speed giving multiple attacks on tied initiative and the random harlot table and the helmet rule - we ignored them. But it's kind of interesting to see the game's author say, yes, aging matters and your stats change as you grow into maturity.
When I read those books, though, I'd already started to play GURPS and Rolemaster. And I'd always felt that narratively those game systems better dealt with Gord's magical dagger, the effect of armor on weapons, characters who push against the grain of a specific class, and more.
Gord gets some special benefits not normally done in AD&D. +1 to hit and damage with shortsword and dagger, no penalty for off hand use of a second weapon (DMG, p. 70). That's a steep bonus, too, negating -2 and -4. So a normal thief would attack at -2 and -4, Gord at +1 and +1 on the two weapons. A +3 and a +5 bonus, net. Yowza.
It's not to say D&D can't handle them, but to handle them you pretty much just have to hand out special benefits and or special restrictions. It's not natively handled, it's a bolt-on per GM judgment. For Gord to be a thief-acrobat he's got to have met acrobats are the right point in his career. To be an excellent sword-and-dagger handler he has to get a special bonus his class wouldn't otherwise give out. All possible, of course, in a "the GM says so" kind of way. But it felt like receive-and-build systems or roll-and-build systems instead of roll-and-choose systems (point buy, roll-class-level-point buy, and class and level, respectively) handled that kind of guy better. So oddly the official writeup of how to do him in Unearthed Arcana era AD&D was something that pushed me to think it would be easier to do him in a system not of the author's choosing. Ironic, I think - the demonstration of how to do it in AD&D convinced me AD&D wasn't the system to do it in.
I've never statted the guy up in GURPS, since I don't expect I'd use him. It's all subjective and especially prone to arguments about how to convert AD&D to GURPS. His dagger, though, or one like it, would be an easy one to make up:
Large Knife (sw+1/cutting, thr+1/impaling, ST 6) with Accuracy +1, Puissance +1, Penetrating Blade (5). The blade can cut through metal, stone, etc. without dulling or wearing beyond normal wear for usage. When used to carve through walls, chests, etc. use thrust+1 cutting damage to represent the difficulty of cutting with the tip of a dagger.