Spiderman is Marvel, no doubt about it. He's got that classic Marvel charm and his storylines are deeply integrated into the larger Marvel comic universe.
It's hard to give an exact age as it depends on the specific storyline and continuity. But generally, Spiderman was likely in his early to mid-twenties in the 1990 comic.
The debut of the first Spiderman comic happened in 1962. It was a significant moment in comic book history and launched the iconic character into the world of superheroes.