I think he's 30 or 31. Anyway, he's around that age.
I don't know if he's 30 or 31 but he must be around that age.
「彼は30歳?31歳?とにかくそれぐらいだよ」
(I think he's 30 or 31. Anyway, he's around that age.)
「よく分からない」時に、「I think he's around ~」や「He must be around ~」、「I'm sure he's around ~」というような表現を使う事が出来ます。
I think he's around 30.
(彼は30歳くらいだと思う。)
He's probably about 30.
(彼は30歳くらいかもしれない。)
He must be in his early 30s.
(彼は30代の後半だろう。)
などの言い方も使えます。
I don't know exactly how old he is, but I think he is around thirty.
He must be thirty or so.
I don't know exactly how old he is, but I think he is around thirty.
「正確に歳は分からないけど、30歳くらいだと思うよ」
「〇〇歳くらい」にはaroundという単語をよく使います。
「〇〇かそれくらい」のように伝えるときによくある別の言い方はor soです。
He must/should be thirty or so.
「彼は30最かそれくらいに違いないよ/それくらいのはずだよ」
ご参考になれば幸いです。