Take it from me, these ages are a lot younger than most SLPs (myself included) are used to. I LOVE that research is showing kids are ready to tackle sounds earlier than we thought. If your kid is struggling with a sound and you are concerned, talk with your local SLP (you can get info from your pediatrician or school district).
It may take the SLP world a while to catch up to this most recent study compiling data regarding speech acquisition. BUT at least now we have a starting point.