Hello there in Nepal! If they are still shedding their baby teeth you can get a very accurate guess. Once adults though it is an educated guess based on the condition of their teeth and other things. I had an English Mastiff once that they said was about 5. She wasnt she was feral for who knows how long. If she was 5 when I got her she was still running and playing at age 12 which usually does not happen with that breed. I suspect she was between 1 and 2 when I got her. Do you have any pictures of the dog in question?
There is an art to this. You can determine puppy aging pretty accurately with milk teeth/adult teeth. You can determine adolescence with body shape and growth rates. Once they're adults, teeth wear can sometimes help figure out in a rough sense whether they're closer to three or six years old, but the kind of care they've had can make the teeth wear prematurely (or stay pearly white longer) but it's a guess. I've seen some dogs labeled with very inaccurate ages by vets based on their teeth because the vets didn't take into account that the dog had been kept on a chain (gnawing metal constantly to try to get free) or was raw fed or got annual dental cleanings (keeping pearly white teeth for years and years). With "most" kibble-fed dogs not getting annual dental cleanings, they get tarter on their back molars around year 3 (you have to lift up the lip to check), and the older they are, the more they accumulate.
You can also figure out with eyes when a dog is over 7--there are changes to the structure of the eye that happen after age 7 in almost all dogs. In intact males, low-hanging testicles can also tell you when a dog is older (over 6).
In rescue work, we use all of of these signs, together, to try to come up with an age range. I find that some vets are very good at it and some are absolutely terrible.
No one can tell you accurately whether a dog is 3, 4, or 5 years old though. You are just looking for ranges based on some of the markers I described.