You cannot be too old for dental implants. The issue is your health. As long as you are a candidate for elective surgery, you can have dental implants. A person with medical conditions such as uncontrolled diabetes, heart disease, severe hypertension etc., would not be an appropriate candidate for dental implants.
With regards to being too young, dental implants are usually done for patients whose jaw bones are fully formed, so that would be late teenage years. If you are not ready yet for implants there are temporary options so you don’t have to go without teeth