Although they are the same age, the Miller twins are taller than THEM.
Them is the answer because of the presence of the word "they" in the sentence. The speaker is in the third person point of view or is an observer. In the above sentence the speaker may be an individual or in singular form.
If the speakers were in the first person point of view, then they are part of the group with the Miller twins, the sentence would have been:
Although we are the same age, the Miller twins are taller than us.
