The 3rd row seats in the X5 is recommended for ppl 5'7" and under. However I have heard that 6' ppl sat back there (not for long periods).

The dealer also said toddlers (5yrs old, even 4yrs old as long as they meet the requirement for boaster seats) can sit back there with a boaster seat.
The logic is if the seat belt is in the correct position right on the shoulder (not the neck).
You can not install infant or toddler child seats in the 3rd row because there are not child seat anchors back there.

I would think the 3rd row area is the safest place to be in, because of all the reinforcement around that area as well as the airbag curtains reaches back there and with minimum window/glass to shatter.
The 2nd row seats also provide frontal support for the 3rd row and the rear truck and bumper crumples on any impact absorbing most or all of the rear end collision.