What is the age limit as per the scriptures of getting a Yajnopavita for the various varnas?
Ideal time to perform the sacred thread ceremony:
Manu Smriti 2.36. In the eighth year after conception, one should perform the initiation (upanayana) of a Brahmana, in the eleventh after conception (that) of a Kshatriya, but in the twelfth that of a Vaisya
The maximum age limits till which sacred thread ceremony can still be performed for the three varnas:
Manu Smriti 2.38. The (time for the) Savitri (initiation) of a Brahmana does not pass until the completion of the sixteenth year (after conception), of a Kshatriya until the completion of the twenty-second, and of a Vaisya until the completion of the twenty-fourth.
And if Upanayana is not performed even within those prescribed time limits, then this is what happens as per Manu:
2.39. After those (periods men of) these three (castes) who have not received the sacrament at the proper time, become Vratyas (outcasts), excluded from the Savitri (initiation) and despised by the Aryans