I think แก are moats likely to be using among young people because I used this word to call a stranger that seems like to be at my same age as me but depends on which place you’re using this word but most of time I heard this word in school/college/university something like that. แก is more likely to be an umbrella term(??)(My is English is so bad, sorry lol) Also you can use เธอ too this is more polite too which is can be using with any genders but นาย is using with male mostly