Why is the prevailing theory of ghosts that they will be frozen in time?

Ghosts aren't visible--they do not reflect nor emit visible light. They don't have an 'appearance' at all. Instead, the image people see is a mental projection. Most spirits are ovoids, spheres, or irregular shaped blobs of energy--not detailed people shapes (that can happen, but it's usually constructs/servitors that have shapes, not ghosts or wild spirits).

The spirit projects an image of itself into your mind, or you pick up on the image of how it sees itself. How do you see yourself? That is how you would 'look' as a ghost.

I don't think everybody thinks that, some people claim to have seen a ghost that died as a child appear as an adult, or alternately appear younger than their age at death.

Anyway, since ghosts are imaginary you can make up any story you like about how they age or not, it's not like anyone can disprove it.

If they were real it would make sense for them not to age since aging is caused by processes in the physical body which of course a ghost would no longer have.

Aging is a biological process, and the presumed "spirit" has left the physical body and therefore has lost that biological aging process.

On the other hand, if they can hear, see, and speak (as witnesses seem to suggest), then why wouldn't they age, too? All the senses are physical processes as well, so they could by that logic, age too.

So it appears as though you have shown us another good reason NOT to believe in ghosts!

I don't think that's the prevailing theory. Elvis Presley appears in different forms depending where he's spotted.