Well I'm 60 and I don't think it's that great. (no kidding) I think I stopped thinking of myself as middle-aged about 5 or 10 years ago. I guess there are positives - but it obviously depends completely on the individual and their circumstances.
I think in many ways my mental health was better when I was younger - and I had a much busier life. My family life was great and my social life was good too. It's sort of depressing when you get older and your looks fade - you become invisible. I wasn't invisible before - I was pretty vain tbh.
I also worry about getting old - actually moreso for my wife. I hope she doesn't get something horrible like Alzheimer's. That would really break my heart to see that.
Edit: I guess the only really good thing is you no longer have obligations, or at least they are a lot less than before. I'm lucky that I have a pretty full life to look back on, plus I have an ex-wife that I'm still close to and a great son. If I had lived a solitary boring life I think I would be very depressed now.