Can ear piercing be done to a 21 year old man in Hinduism?

Ear-piercing or Karnavedha is considered as one of the most important Hindu Samskaras performed for a child during sixth/seventh month or third/fifth year after his birth. 

This ritual is usually performed in even years after the birth of the child
because performing it in odd years is considered ill-fated. 

A special function is organised and a very auspicious day is chosen for the ear piercing ceremony where puja is done under sunlight and then earrings are given to the child.

So the simple answer to your question is yes you can ear piercing be done to a 21 year old man in Hinduism.

Ear piercing is not specific to a particular gender but it quite common among both the male and females in India. 

Usually for a girl child the first ear pierced is the left ear whereas for a boy child the first ear pierced is the right. The people believe that by means of doing this it helps in opening of the inner ears of the child so as to receive the sacred sounds. 

The ear piercing ritual has more of spiritual fascination and emblematic significance. The people believe that by hearing these sacred sounds or the mantras which helps in flushing our sin and caring and protecting our spirit.

BENEFITS OF EAR-PIERCING

Ear piercing is believed to regulate the menstrual cycle of a girl.

It is also believed to maintain the flow of electric current in the body.

It also helps in getting rid of hysteria and other diseases.

NOSE-PIERCING

Nose piercing is another important ritual performed in Hindus where a married Hindu women wears Nath .Nostril piercing is very common among Hindu women  and it is believed that it makes the childbirth easier. And generally nose piercing is regarded as a sign of beauty and it is done for the  purpose of wearing jewellery.

According to a medieval writer, “All the accumulated merits disappear at the sight of a Brahmana through whose ear holes do not pass the rays of the sun. No gift should be given to him in the Sraddha ceremonies. If one gives, he becomes an `asura` or ‘demon’.”

But owing to modern influences, ear or nose piercing have become scarce and is fading out as a ritual as its performed by very few people . Apart from that, one can notice young girls flaunting studs instead of traditional Naths.