If you just want to instruct then doing the ATPL exams will be pointless as you will never get the 500 multi-crew hours needed to get the license issued. However if you do want to go to the airlines then it is essential to take them. 39 is not too old. I was older than that when I got my first full time job, and I know of one pilot who got his first job (on a jet) after only instructing experience at age 50.