For my DD, this type of problem was not unusual. I believe our math curriculum used this type of thinking fairly early on, though personally I don't think I would consider it to be learning algebra.
I actually have strong memories of the trapezoid/parallelogram/rhombus unit because it was a particularly good one, where DD worked in a small group where they were asked to derive the formulas, based on what they had learned about other shapes. She loved it, and still talks about it.