Did people age faster back then?
They made future Marty and Jennifer look like they were 60-70 more than 47
share1. Yes, people tended to look and appear, or even act older (by today's standards) back then, than how we think of it today. It was a combination of societal culture, clothing style, and hair styles.
2. The idea of BTTF2 2015 was to go silly and extreme. They didn't actually think 2015 was literally going to look like that. So in the case of aging up MJF and Shue, they would just figure why not make them look so much different than their actual selves. Make them more of a caricatures of adult selves to go along with the "silly and extreme" motif of that part of the movie