Lots of options as listed above.
I am nearing your age, quicker than I would like.
There are so many factors to take into account. Why do you want a bike? To tear it up, to ride in a paddock, to jump buses, to impress your children and their friends, to try to help your partner collect on your life insurance etc.
Basically there are crf50 style bikes which are all based on the same frame geometry although some may have an added couple of inches in length. You then have crf70 style bikes which are a more mid size mini. There are others but to be honest I still get confused by the myriad of styles and adaptations of styles available.
If you are of a reasonable size I would suggest a crf70 style bike. However, there are plenty of larger human beings riding stock class crf50's with varying amounts of success.
Many different bike retailers will promise you the earth and deliver nothing more than a cheap 2 dimensional Nauru. It may look O.K. but cannot be seen as a solution. Sorry, I am getting off track.
Several dealers sell well tested bikes with good to excellent after sales service.
DHZ.
Pitster Pro( also sold by DHZ)
PSTO
Ciniwerx
This is by no means a definitive list of quality Mini/Pit bike retailers but they would be a good place to start and would give you an idea of what is available at a given price point.
Best of luck with your purchase. Take as much time as you need and look through the forums, most of questions you may have will have been answered at some stage.
Logic