The word Upanayan consists of two words, ‘upa’ & ‘nayana’. ‘Upa’ means near also ‘nayana’ means to taking towards. Upanayan means taking one towards the Guru tatva.
#SanatanaDharma
A Kshatriya - By age 12
A Vaisya - By age 16.
If by any chance they miss , the upper limit is 24 for performing the sanskara.
Just brief, These is lot more to this