You have to be at least 16 to start an engineering apprenticeship. Most commonly I meet engineers who started work at 21 when they left University. The oldest working engineer I know is in his late 60s. Engineers build up really good experience over their lives, and the job isn’t physically difficult, so you can do it at any age as long as your brain is in decent working order. So I know quite a few engineers who have “retired” but then still give advice for 2 days a week or something like that, because they still enjoy it.
You can retrain as an engineer later in life as a career change, it’s likely to take 3-4 years.