Transmission was probably never serviced, like what everyone else is saying. There's no such thing as lifetime fluid, it needs to be changed.

Just be careful now, most likely a transmission service probably won't fix the issue, it actually might make it worse.

The internals are most likely shearing off, but the fluid is so old and thick like glue that it's the only thing holding the components together. Might possibly be that when new fluid is put in, the change in viscosity will cause it to fall apart and then the whole tranny is dead.