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.
Why is it obviously not the case, given that the only person who did portray Data did age at a human rate?