If the person was born in hospital, the hospital will have a record of the birth and the hospital will provide  a letter in English stating the birth date. I was born in Prasuti griha in the 70s and they have my birth record. This letter is enough.

If the person was born in the house, that person can go to the ward or VDC office and they will provide a letter stating that the person was born on that date in the house (address) to so and so parents. In Kathmandu, bhaktapur and Lalitpur, the ward office can provide you a letter in English so you don't have to translate it. This letter is enough.

I think most of the big cities wards will provide a letter in English now a days. I don't know about VDCs.