There's no need for a phrase for it. The others are based on the first word of their age and refers to a decade. The problem with saying "in his hundreds" is that it doesn't narrow it down to a decade. It could lead to someone thinking that the person could be 120, 260, 490 - those all start with a multiple of a hundred.
When someone is aged between 1 and 10, we don't say they're "in their" anything, the same applies when they're between 10 and 12, between 13 and 19 they're teenagers, then comes "in their twenties/thirties/forties/fifties/sixties/seventies/eighties/nineties".
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