In principle you are never too old for an implant. Implanting is possible provided that you are healthy and that you are able to physically withstand the treatment. It is regularly used with success for people aged 90+.
People can be too young for implants. If a tooth is to be replaced whereby the gums are visible (specifically in relation to the top teeth) then it can be better not to have an implant before the age of 18. Up to that age it is wise to replace teeth or molars with a temporary structure.
The jaws must be fully developped and not all people achieve this by the same age. If implants are fitted too early the implant will remain "fixed" in its position, while the other teeth develop even further. As a result of this the implant will ultimately remain above the level of the adjacent teeth. This is quite unattractive and very little can be done about it subsequently.