The R sound is one of the hardest sounds for a person to learn. 90% of kids have learned the sound by the age of 8.
The L sound is another. 90% of kids have learned the sound by the age of 7.
The TH sound is difficult too, as is the J sound. (I didn't ask about X, but I'm curious).
The M and N sounds are easier. As are B, C, D and all of the other sounds Oliver is using like crazy right now. Lately, he's fond of saying, "Jess, what doin?"