In everyday society, when speaking to an unknown person, Thai people use Thai word phi because they don't know if this person is older or not. Even between friends, there is a hierarchy. The younger person will use phi word. The elder person will use the first name. Everything is a question of age and sex.
Seniority is a important part of Thai society.
A younger person shall respect the older person. Thai children used to live not far from their parents. A big part of the wages should be given to parents. Now things are changing. With the industrialization of Thailand, a lot of young Thai people go to Bangkok to take their chance.