A good mechanical watch can easily last 20yrs even with some abuse. Those high end would even keep or appreciate their value with proper care.
A quality mechanical watch will easily last over a lifetime, provided it isn't abused; and if it is properly serviced (about once every 5 to 7 years). It's the last part where many watch owners who aren't heavy into watches usually screw up. They think that the lack of a battery means the case-back never needs to be opened up. And that if a mechanical stops working completely, then it has completely worn out and reached the end of its service life.
Recently read a heart-warming story on the biggest watch forum about a son who restored his dad's old Rolex, and presented him with his old friend. The father could never afford a Rolex on his own. He received it as a wedding gift from his parents-in-Law. He was shocked & happy! Wore the watch for the better part of two decades. Wore it doing the type of dirty jobs that would make probably every Rolex owner cringe. One day, it completely stopped working. He placed it back in its original box, and then put it away into the back of a drawer. Wore a much less expensive watch after that.
He wasn't a watch collector or even an enthusiast. He was just an average guy who simply knew that Rolex meant quality. Ironically, he was a victim of the legendary mystique surrounding the brand. Like many, he just assumed that owning a Rolex meant never needing to get the watch serviced. It was supposed to be
that good. After all, it's a Rolex. Even though Rolex is a quality luxury brand, it doesn't work that way. Even a BMW M5 needs servicing. (Ironically, even more so than a car that isn't built for performance.) So, when the father's Rolex finally gave out, he just assumed that it had given everything it had and that there was no conceivable way to fix it. Thankfully, his son knew better. By this time a routine servicing wasn't enough. After about 20 years, the watch needed to be severely over-hauled. It was, and the father got his Rolex back.
A mechanical watch, properly maintained, is the sort of thing a young man can pass down to a future son after the man is old & grey. 20 years is about right for a quartz watch with a battery in it. The only problem with passing down a mechanical watch is if parts are no longer available for it. But there are ways around that. Rolex stocks parts for vintage watches years afterwards. Patek Philippe will actually make a needed part for a customer (if necessary). Granted, those are very expensive options. A less expensive one is to get a mechanical that uses an ETA 2824 movement, and make sure that the brand is part of the Swatch Group.