人間の平均寿命が100歳になったとしたら

人間の平均寿命が100歳になったとしたら、人々は老後にどのようなことをしたいと考えるだろうか。

Hi. Would you explain why なった, instead of なる, is used here when humanity's average life span reaching 100 is in the future?

Thank you.

Because Japanese "past tense" isn't really the past (tense) but rather the perfect (aspect). The important thing isn't whether the action in question happened/happens in the past or the future relative to now, but whether or not the action has been completed at the time in question.

In this hypothetical situation where humanity's life span has reached one hundred, by the time they're thinking about what to in old age their life span will have already reached that point. If you said "なるとしたら", the nuance would be that you're asking what they're going to be thinking about this at a time when the life span is going to reach 100 (but hasn't already).

As bentenmusume-san explained, なるとしたら is for the case where the life span is going to reach 100 (but hasn't already), so the clause following it expresses an event/a thing before that time.
cf.
人間の平均寿命が100歳になるとしたら、人々は何に備えなければならないだろうか。

日本に行くとしたら、船で行きたい。
日本に行ったとしたら、新幹線に乗ってみたい。

Toritoribe said:

As bentenmusume-san explained, なるとしたら is for the case where the life span is going to reach 100 (but hasn't already), so the clause following it expresses an event/a thing before that time.
cf.
人間の平均寿命が100歳になるとしたら、人々は何に備えなければならないだろうか。

日本に行くとしたら、船で行きたい。
日本に行ったとしたら、新幹線に乗ってみたい。

Click to expand...

Thank you both!
So if た form is used before としたら, then the clause following としたら expresses an event/a thing after that time, right?

Yes, you got it right!