How old to play The Escapists 2?

Hard to say... Contrary to Starbound Terraria doesn't have a story but rather a sequence of bosses (about a dozen in normal mode) you have to to kill while progressing in the tech tree and after you have beaten the final boss you can move on to Hard mode and then the game begins.

Biomes are much more sophisticated and feel more natural in Terraria (Starbound barely has any Biome specific loot) but on the other hand Starbound has multiple planets, space travel including upgrading your spaceship, and most importantly a survial system where you regularly have to eat, can harvest and plant produce and breed animals, cook and build your own towns and fill them with lots of different alien species. Building is better in Terraria although Starbound brought some good new ideas to the table like using the same blocks for front and background while Terraria has seperate blocks (front) and walls (background).

I played both of them for several hundred hours and enjoyed them a lot but I liked Terraria a tiny bit more.

Indeed, Sundance85 explained it very well. I'd also add a few additional notes:

In terms of theme, Terraria is a fantasy game where you're playing on a specific world, while Starbound explores the space setting. That may make Starbound seem more vast, but it's not quite so. A large map on Terraria will give you hundreds of hours of stuff to explore... and while always different, each Starbound planet doesn't have that much to do in it (hundreds of planets to explore, but they're variations of only a handful of biomes). Terraria wins this point, in my opinion.

That said, Starbound has a technological theme that allows for things that Terraria doesn't have (machines for multiple purposes, vehicles & customizable mechs, space stations, commerce and bounty hunting across galaxies, etc.). Terraria has sets of artifacts (and items that can only be acquired through probability of being dropped by specific monsters), and vast crafting trees... but no customizable mechs ;) They're both good, in different ways.

Finally, Starbound felt a bit more "grindy" to me... slower to mine resources, larger effort to find rare materials, slower to progress overall. Quests span across galaxies, leaving you sometimes lost on what you need to do.

So, like Sundance, personally I enjoyed Terraria a bit more, too. Overall, if your cousin likes Starbound he'll very likely love Terraria (and will probably rage-quit a few times when he realizes how frustrating some boss battles can be).