Agree with Rusty.
tus is a general command "exclamation (a un perro),,,Good boy! Here boy!"
Lets change it to Seat! (¡Sientate!) for example.
So the modism is
"A perro viejo no hay 'Sientate!¡Sientate!"For an old dog, there are no Seat!.
if your dog is old, s/he no longer receives such commands.
You can’t teach an old dog new tricks.
saludos
PD for example to cats it's usual to say "miz,miz" to come to you (with thousand of variations "misu, misu", "misina,misina", "miss,miss"...)