Age

You will know either when it breaks and fixing it would cost almost as much as a new computer or when it will no longer run the software you need or perform the tasks that you need it to perform. Or both.

Elainemeyer wrote:

It’s 2015 or 2016 I am not technically inclined so I won’t know if it updates or not

But you will know if it stops working. Or if applications you use stop working. That requires very little technical knowledge.

But, if it were me and I had the money, I'd buy a new one.

It could be either. Or both. Do other things stay connected? Have you tried unplugging the router, waiting five minutes and plugging it back in?

You can contact Apple and arrange to have them run diagnostics. They can often do that right over the phone.

Genius Bar Reservation and Apple Support Options - Apple

If they find something wrong, and it were my computer, I wouldn't spend money to get it repaired unless buying a new computer were out of the question.

Elainemeyer wrote:

Was going to buy a new one , I tried unplugging router still does the same thing . Not staying connected. I don’t want to get a new computer if it continues not staying connected

If nothing stays connected the router, the first thing you should do is get that resolved. Start by contacting your internet service provider.

Wi-Fi does not get old. You almost certainly have a different issue (than age).

We almost never recommend a computer be replaced because it's Wi-Fi is too old.