If he is still alive today, he…

Suppose Jeff was born on 1/1/1982. He went missing a long time ago. At the present time, he could be either dead or alive, but nobody knows for sure:

1 "If Jeff is still alive today, he is 30 years old."
2 "If Jeff is still alive today, he will be 30 years old."
3 "If Jeff is still alive today, he would be 30 years old."

Which one is standard English?

The only one that is correct in the scenario you present is #1:

If Jeff is still alive today, he is 30 years old.

The is in the first clause corresponds to the is in the second.

I'd say that numbers 2 and 3 (If Jeff is still alive today, he will be 30 years old and If Jeff is still alive today, he would be 30 years old) would work in speech, but not in writing. The will be and would be give a sense of speculation, but grammatically they do not correspond to the is in the first clause.

The most common form is "If X is [true/whatever], Y is [true/whatever]".

But "If X is [true/whatever], Y will be [true/whatever]" is neither invalid, nor uncommon. You can interpret such use of "future tense" as implying that Y follows X (logically, or temporally, it makes no difference). Alternatively, see it as meaning that Y will be found to be true if/when you come to examine the matter closely.

OP's third version does occur, but it's a non-standard mixture of tenses. The "correct" version using would is "If Jeff were [to be] still alive today, he would be 30 years old". It's an example of future subjunctive, which I believe is a declining usage; non-native speakers can generally ignore it.