Lol idk if you're just rolling with the meme but yeah it would be used as a compliment, but it's unfortunately a very unintended double entendre here. I think the other meaning came from a peg board, and it's like a "I would have placed you..." thought. "I would have pegged you as burger kind of guy" for example, if someone orders a salad at a restaurant and you were surprised by that choice.
She's saying "I would assume that he's in his 70's". A better way to phrase it would've been "I would've pegged him to be in his 70's" but even that sounds dirty because we're all degenerates
"I would have pegged him at 70" means "I would have identified him to be 70 years old," however due to modern slang it appears to be saying "I would have used a strap-on in a sexual act with him 30 years ago."
Allegedly, the older use of "peg" comes from using a wooden peg to make a door not close properly, so you can come back later and gain entry illicitly.
The modern version is conceptually similar but a very different experience.