Yes. Being "mature for 14" doesn't make a lick of difference. 17 is only one year from being an adult, and I know that at 17 I was "experienced" and 14 year olds should not be. In simple terms, no it's not okay for a high schooler to date a middle schooler.
Im 15 and I was talking to a 19 year old (he was a family friend) haha at the time I thought it was alright but now I think it's too much! You only have a 3 year difference.. I don't think it's that bad as long as you guys aren't having sex