Hey there Owen,
The only real age restriction to transitioning would be your health and then health issues affect every ones transition or their ability to transition regardless of age. The best place to start would be to get a complete physical from your doctor, including blood work. HRT can have a huge impact on the body, and again it depends on the individual. Some don't experience any ill effects while for others it can be fatal. Those at risk tend to be those who are at risk for heart disease, those who have other coronary complications. High cholesterol levels can be a factor that should be addressed before starting HRT.
From complications caused by HRT to those that can complicate GRS/SRS you can see there is a wide spectrum of issues that you should be aware of. As I mentioned some experience no ill effects while other do. Age can be factor but not always I'm 54 and I'm OK so far.
Visit the Wiki and read the HRT Article found
here. While this article should
NOT be used to replace competent medical advice it is fairly complete and accurate and it lists the possible side effects that could be expected.
Steph