i used to let this happen when i first came here. now not a chance. i care not if it is an old or young person i call them out every single time. i find that younger staff are changing with this as well though. a few times i have had a younger thai tell a budding customer to wait as i was there first.
make your voice heard in these instances regardless of how thai feel. stand with your own convictions of what you feel is disrespectful.
this is plain rude no matter the culture or what the locals feel or believe.