Should I use past tense of the verb in the following sentence?

Question

I thought my personality would change when I reached/ reach the age of twenty.

Answer 1

You should say:

I thought my personality would have changed when I had reached the age of twenty.

Which is 3rd conditional, and that because using thought you imply a past event, which cannot be changed.

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Let's say you are under the age of twenty, then you should say:

I think my personality will change when I reach the age of twenty

which is the 1st conditional.

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Your expression is the 2nd conditional, let's say you are under the age of twenty, and believe you will die before your birthday:

I think my personality would change, if I reached the age of twenty