Have you reached a stage in your career where you can go further, or has it flattened out? Are there lots of exciting challenges ahead, or would it be more of the same?
If there's still scope for excitement and fresh chapters, then carry on with it. If not, then surely you have loads to pass on to others, and then teaching would be perfect.
I've recently got into a bit of teaching, and it's been fantastic. After years of flitting from one thing to another, and never really settling, I wish I'd got into it sooner. I've found it far more rewarding than most of the other stuff I've done. And it's so much fun! Work? It don't feel like it! something to actually look forward to, as opposed to the tediousness of something that's always the same thing.
You may be a fantastic mechanic/engineer. You could be a fantastic teacher.
As for being a 'has-been', well, that's a load of bollocks. You'll be able to look back on things, and think 'well, I've been a teacher'.
And there are few more important jobs in our society, than those who educate others.
Do it.