You are eligible for NET assistant professorship if you completed your master's degree in Any of the 93 subjects offered. Lectureship posts are not bound to any age limit, For JRF you need to be under 28 years. Refer this link for eligibility criteria. https://www.eduncle.com/cbse-ugc-net-exam.php

The age limit for net exam is not more than 28 years as on 1st June of the year of admission. As you are 40 years old you can't write the net exam.
For details about NET click here