Old dog:

Old dog is of the negative traits.
It has a value of -4

The description is as follows:

Old dog:
You have trouble learning new tricks. Your perk rate is reduced by one. Normally that would mean, that as a human you now receive a perk every 4 experience levels and for Ghouls and Super Mutants every 5. Should another trait have changed your perk rate already, this is added to that change.