MMA training has no age limits to speak of per say. You can train at any suitable age. To sign up for an MMA fight though does have an age restriction. Every states laws vary, some say 16, others say 18. So I'd look up your states laws concerning that.

Same as any other sport. I think 7 or 8 is a good age to kids involved in athletics of any kind and MMA is the same. Although I do think kids will have to do one particular discipline at a time. Too hard to do all aspects at a young age and at the age of less than about 12 or 13, I don't think many kids and focus and train in all aspects of MMA. Focusing on one of the main skills needed is best ala BJJ, Muay Thai, or Wrestling until they're older and in their mid-late teens.

You need to start at the age of 4 years old, If you don't start young you will never have a shot.

You have to retire by the time your 40. The techniques in mma are for the young and not for old farts.

Its never too late to start training, however if you aim to become professional, many pro's started in their teens with dedicated training since then. If you start now and dedicate yourself several days per week you should be ready to compete within several years.