My kids have first check up at 3. I still remeber the dentist just use q-tip to count my daughter teeth cause she's really scare & keep crying. My son who is more brave & dentist already use some tool to clean his tooth.at the first time. I think it's depend on the dentist' view & the kids condition for the treatment.
Regarding full mouth x-ray, my kids only take the front tooth rather than full mouth at the first visit. My daugthers took full mouth x-ray not until she's 5 & my son took it at 4. hope it help.