My children have started playing age 5 and I think is appropriate, although not online.
I have been able to play with them, my children can all play together with the same roleplay as with physical toys and we can turn it off immediately.
It is simple enough to play and go on adventures, build villages and kill monsters which is no more violent than bashing two plastic figures together.
In survival mode it introduces the need to prepare, explore and gather resources (which can be time consuming to achieve). Also rearing animals for meat, eggs, wool, leather and growing crops on the correct soil and with water. There are different habitats too.
You can play in peaceful mode so no hostile creatures at all.
It lets them be creative and do what they want without seeking permission and still experience consequence.
My only caution would be that my son has become too keen on it so we will have to have a break.
Also the sounds can be spooky but they even affect me as an adult! Easy to turn the monsters off/sound off.