I'd say any time that anyone does anything that doesn't come naturally with the intent to fit in, it's masking. I'd also say that everyone does it, it's masking, regardless of whether they are diagnosed with ASD, something else, or nothing at all.
If someone tries to start a conversation and I don't feel comfortable responding but I find a way to do it anyway, that's masking. If I choose to go to a party that I'd rather not go to, I'd also say that's a form of masking.
If an NT who hates playing cards chooses to play cards without complaining because it's what the group wants to do, and pretends to enjoy it even though he or she doesn't really enjoy it, I'd also call that a form of masking.
Some people mask more than others, and some are better at it than others, but I'd say there are very few people who are older than 2 years old who don't mask at all.
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Level 1 Autism Spectrum Disorder / Asperger's Syndrome.