It's for 12 and under. The first several times we went, there was no posted age, and my kids were younger then (probably 3-6 and 5-8) and they had a blast. When we went when they were 9-12, they still had a really good time (well, except for the fact that it was 58 degrees at the time!

). There's just a lot of fun stuff.
Last trip they were 12-15, and although we walked over there once, they had signs posted everywhere saying 12 and under and seemed to be enforcing it. My youngest didn't want to go by himself, so we just stayed away. This was mid-summer though, so it was definitely crowded. I think at a less crowded time, they'd probably give the kids the benefit of the doubt.
Only thing is the "hang on to a bar and slide" ride - not sure what its name really is, does have a 5 foot height limit, so you could definitely be 12 and over that and not allowed to ride just that one ride. For the most part, it's not like you have to line up for anything so you could certainly go and check it out and if they're bored in 5 minutes, leave!
We're in Wisconsin and go to WI Dells at least once a summer and they have a water park here with a similar area but open to all ages and even DH and I have a good time doing similar rides, slides, cargo net/icebergs, etc. so I'd think they'd enjoy it!