No one ever said he aged like a normal human.
The line was a comparison between Data and Juliana. Juliana, like Data,
will grow old just like the real Juliana would have, the implication being that Data will also grow old at a human rate. But that's obviously not the case. So it's more of a bad choice of dialogue more than anything else.
Definately a bad choice of dialogue seeing as in future of "All Good Things" everyone has aged but data still looks the same.