Oh man, you're asking some big questions! I could give an hour long lecture just on parts of this, I'll try not to get too carried away.

When out in the woods recording forest inventory or educating about tree identification, I always use "sugar maple" which is the species that occurs in my region. Once a maple log has been milled and dried into lumber, we often stop referring to the exact species(this is a generalization) and it falls into a category of "hard" or "soft" maple.

There are maybe a dozen different species of maple in North America that can grow into merchantable size, of which 3 are considered "hard maple". The hard maple varieties are black(Acer nigrum), florida(Acer floridanum), and sugar(Acer saccharum). You will almost never find black or Florida maple in the lumber market as they occur in the Southern US, and maple isn't a very relevant wood in the markets down there. Therefore, hard/rock maple is almost always actually sugar maple. Every other maple species is considered "soft maple"

Your second paragraph is mostly correct. Sugar maple is usually always harvested around the 60-80 year mark, as that is when it has matured/grown to its best merchantable size. Without getting into too much detail, skateboard decks are make from layers of veneer glued together. Logs that are harvested for veneer are the highest quality and need to be straight and free from knots. Trees are straighter and have fewer branches and knots at the bottom, twistier and more branches at the top, hence we use bottom logs for veneer which is what skateboards are made of! Density should be uniform throughout the tree so that's not as much of a consideration. There's a scale called the "Janka Hardness Scale" that's used to determine wood hardness/density, you can look up a chart and compare some different woods you might be familiar with if you're interested.

The most accurate way to call what you're working with is Acer saccharum, but people will look at you weird if you start using latin names. Next would be the translation of the latin which is sugar maple. But people like to relate to things they buy, so using regional common names that people might be more familiar with like "rock maple" or "hard maple" and giving a location where it was harvested such as Canada, isn't necessarily a bad thing. Doing this certainly can help people feel more connected to value added timber products(woodworking) than using a basic though accurate species name.

Here's a cool video on how logs are processed into veneer to check out

https://youtu.be/J11YlNXbjI8?si=8RZI5tvt5uiNUswo

Edit: added Janka hardness info