In 1990 a person is 15 years old. in 1995 the same person is 10 years old. how is this possible?

It is possible if it is 1990 BC and 1995 BC.

Then the person who is 10 years old in 1995 BC after 5 years (in 1990 BC) is 15 years old:

1995 BC + 5 years = 1990 BC

10 years + 5 years = 15 years