Is 上下 always interchangeable with 左右?
Mostly,上下 and 左右 have the same mean when you describe something.

When you describe something high or low(mostly abstract things,like level), you can use 左右,比如 他的水平很高，有十级左右了。Abstract things usually use 左右, like level, time(十分钟左右,how long is your own feel). And not say 十分钟上下.

When you describe something big or small,young or old(mostly you can tell by eyes) you can use 上下,比如，他看起来有十岁上下.But, you can also say, 他看起来十岁左右.

Usually, they are quite similar, just remember how people say and use in that way is ok.