Hardwiring was probably a bad way to put it.
The intention was this:
Learning a new skill requires plasticity of the white matter, so that the connections associated with mastering that skill can be forged. Muscle memory is an example of this. As people get older, the rate of whitematter production, coupled with a finite volume in the cranium would require that old connections be deleted, and new ones created with far more viggor than is seen in current brain evaluations.
Eventually the subject will not be able to learn anything new otherwise after a certain age.
Whitematter connections to the hypocampus and thalamus are associated with the correlation, processing, and storage of sensory information, and as such plays
a fundamental role in memory retention. The chemical chains in the greymatter associated with memories may be present, but the whitematter fibers permitting correlation of that information will become increasingly difficult to create without radically changing the cellular activities of the immortal's brain.
This is similar to what is seen in alzheimer's patients, and is why canabis can actually help them with their dimentia to a limited degree. (Canabinol found in weed stimulates the retrograde signalling pathway which is involved in axon formation and long term memory potentiation. The action of the substance on the diseased white matter causes new pathways to form, and still healthy ones to learn new tricks.)