I’d say the majority of their users are probably older. People who started using it based on familiarity from when they first used it a decade ago (or more). I do know my in-laws refuse to use Google/Apple Maps, because it’s “what they know”.
One time someone gave me an old name for a road and Mapquest was the only place that had it. Cool story, I know.
I used their API for several years for a project to get reverse geocoding to turn city names into lat/lon pairs and vice versa.
Like Mapbox, I assume they have a profitable business selling their mapping services to other companies.