oh, it's not they haven't "mounted" an immune response to vaccinations, it's that vaccines DO wear off. Consider in people- they recommend you get a booster of tetanus vaccine every 10 years, because if you miss it, there's a chance your immune response to a tetanus infection won't be strong enough to save your life. Or whooping cough- we've had epidemics of whooping cough in older teens/ adults because the immunity conferred by the childhood shots has gone away. We mostly find out these vaccine duration problems the hard way, by people getting sick and dying.
I mean, let's be honest, if in the last 9+ years the animal hasn't been able to mount an appropiate immue response to vaccinations what on earth makes you think they'll be able to as a "senior citizen"?