I would say both, as there are players here who are <10 and >60. Some like the detective aspect of solving puzzles like breeding. Others like collecting bugs or finding all the magic plants. Some hard-core players set higher goals such as breeding all the plants, or climbing their way to the top of the leaderboard.

Mainly the plant requirements to stay healthy are more stringent. For example, certain plants can live in level 1 soil long enough to pollinate on the EASY setting. On the NORMAL and HARD setting, they require level 2 soil and will die long before being mature in level 1 soil.