Birdwatcher4, I am 64, husband is 68. Lots of people older than us on the trip. Ours included three or four internal flights, avoiding excessively long coach trips. Even so, this holiday is tiring, several early morning starts but you have to accept that if you want to see the countries. Riviera provided a rep who was with throughout the tour plus local guides each day.
We went in February, landed in Hanoi where it was cool and wet. Much warmer further south. All the accommodation was excellent, the hotel in Siem Reap was fabulous.