Tact, Anyone?

For the love of God, if Person A says to you, "My daughter is ten months old, and she still can't wave or clap her hands. I'm starting to get a little worried," do NOT say to her:

"Don't worry, my son couldn't wave or clap until he was fourteen months old..."

[Person A: huge sigh of relief]

"...but of course, he's mildly retarded."