For early/low 20s
I'm not very sure about the EXACT ranges (if any) that "early 20s" and "low 20s" refer to in English.
But in Chinese, you may at least have three options to choose from, according to "how early/low" the range that you intend to express is:
e.g. A weather forecast broadcast by CCTV in which "5度出头" was used at 1:01, another one in which "20多度" was used at 0:45
There must be some disagreement among native speakers as to what the exact ranges the expressions above refer to (they are not rigidly defined), but I believe generally one will feel that the range gradually turns narrower from left to right. (e.g.
20多岁 → 21 to 25;
20来岁 → 20 to 23；
20岁出头 → 20 to 22 or even lower）
Besides, the patterns mentioned above are fairly productive in Chinese:
multiples of ten + 多/来 + unit
e.g. 30多天, 20来吨 (We don't say 32多天 or 25来吨)
number (it doesn't have to be multiples of ten, but the nuance is a bit tricky, see exmaples below) + unit + 出头
25米出头(a little bit longer than 25m, must be shorter than 26m, since the scale is 25m - 26m),
20米出头(a little bit longer than 20m, can be even longer than 21m, since the scale is 20m - 30m).
For late/high 20s
I agree with Marc that
将近30度/岁 or simply
近30度/岁 (in literal: near 30) can be used.
For mid 20s
I agree with Marc that there may not be a handy phrase for that. However, I think you can alternatively say
25度/岁上下 (in literal: around 25).