A: Physically, easy. I suspect the majority of computers can physically outlive their owner.
Realistically, no. Apple (as well as Microsoft and other vendors) will continue to provide system updates for 5–7 years. Once your computer is too old to receive system and application updates, it is HIGHLY vulnerable to malware and breach.
This puts useful lifespan to around 5–7 years.
An unasked question is is it worth it to keep a computer 10 years?
If the computer is used in a business or otherwise make money, I don’t see a way for an older computer to be profitable, or “worth” keeping around. Around 15 years back I created a program that calculated the cost/benefit of a computer versus purchasing a new computer. I used this to provide hard numbers to clients. In almost every case, if the current computer was two years or older, it was the more responsible choice to replace it with a new computer. In addition to getting a new sparkly, the company almost completely eliminates technical support costs, has little to no support-related downtime, no need to pay for extended warranty, and the user can be more productive.
If the computer is not used for business or make money, and the user doesn’t mind operating in the slow lane, as long as the computer receives OS and app updates, go for it.