I live in Thailand and it's mandated that everyone over 2 wears a mask when out in public (even outdoors) except when eating or drinking. It's enforced as much by social expectation as anything else, I've never seen anyone TELL anyone to wear a mask in the street, but it is the done thing. You certainly can't enter a shop without one but how strict it 'feels' has changed with the ups and downs of each wave.
My youngest was born into covid and seen everyone around her, including siblings and other small children, wearing masks. She actively sought out wearing one by the time she was 18 months old. We have let her as she will probably need to wear them when she turns 2 (masks will not go away quickly in SE Asia!)
She just has thin paper ones and can easily take them off herself, but you'd be surprised how well a very young child can wear a mask if that's all they've ever known. I'm not saying it's right (it's pretty sad that she knows the word 'mask' when she is just learning to speak!) but in answer to the OP, yes, a 2 year old CAN wear a mask but I think it would be very difficult to make a child who isn't used to it suddenly start wearing one. If you want to use one on a flight, definitely introduce it earlier, make it something to play with and reward proper wearing etc. I don't know how to advise really, as my children have all worn them when out and about throughout covid and all the children here just learnt together and improved with practice!
For me, there is no point worrying about whether my children should or shouldn't be wearing one. It is just life here. They had never worn a mask pre-covid (neither had I) but they have to wear one if they want to leave the house, to go to school etc. We are lucky that they are able to play outside without masks, as we agreed in our compound to accept this risk as a group.
It isn't child abuse 
Even before covid it was culturally normal here for small children to wear masks to protect against pollution when the air quality was especially bad, and also if they're just under the weather. It might seem alien to Brits but here, we are the ones considered gross and disrespectful if we DON'T wear a mask when we have a cold or whatever.