Which preposition, for or to, is correct in below sentence?
It is less stressful [for/to] a child than an adult to learn a foreign language.
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The preposition for introduces the person who feels stressful, regardless of the verb form used:
Working is stressful for some.
It is stressful for some to work.
You can omit the subject of the stress:
It is stressful to work.
NGrams shows that to is also used but for is the preferred preposition. It also shows that stress was popularised in the late 20th century.