I wouldn't worry mate. I'm forty seven this year. I've gone through middle age and come out the other side as a born again teenager. I think as you get older everything becomes easier - I say exactly what I think, I don't have any major responsibilities (except the koi pond of course!) and I'm probably fitter than when I was 30. The only downside is that I think I'll have to keep working until I'm 95 because by the time I'm supposed to retire my pension will be worth b****r all and I'll have huge power bills for my 100,000 gal heated retirement pond.
I don't think physical age has got anything to do with it. Middle age starts as soon as you get a mortgage, wife and kids; not necessarily in that order. Suddenly you have a large moral and economic weight on shoulders and less/no choice about what you do with your time. Then suddenly you wake up one day and realise that you've become your parents.
I must admit I spend more time now doing things I did in my 'teens and twenties (apart from the excessive drinking - nearest pub is about 4 miles away!). I seem to have more time to pursue interests and hobbies than I did between 25-40. Occasionally I get reigned in by the missus but we've reached the point now where she realises that for the most part life is easier if she humour's me. In fact the only thing she wasn't keen on was the koi pond. Got around that by letting her have a go at some flukes with the microscope - for some weird reason that seemed to pique her interest.
I find it helps if you don't look in the mirror after forty - you can pretend you haven't aged that much. Only problem is the barber, they now ask if I want my ears and eyebrows trimmed!