How old are David and Pheobe when they finally meet each other in The Memory Keeper's Daughter?

I'm assuming you mean Paul and Phoebe. Technically, David sees Phoebe in the hospital before he sends her away with the nurse after her birth, but he is then unable to ever meet with her again, because he perishes from a heart attack several months before the truth is revealed to Norah.
However, Phoebe is eventually able to meet her twin, Paul, when the two of them are twenty-five. Caroline, the woman who has adopted Phoebe after David gave the girl to her in the hospital, decides to do the right thing and goes to meet Norah, letting her know that her daughter never died. At this point, Norah and Paul make the journey to Pittsburgh, and Paul is eventually able to meet his twin sister.