How low is low? In an older cat, low WBC wouldn't really suggest much to me
If it's just below range, ehhh probably nothing. Also, sometimes CBC's can be skewed a bit depending on how the blood was handled. So, it is probably nothing especially if the vet didn't mention anything. Sometimes low lymphocytes can be a sign of certain cancers, but it all depends on what the cats symptoms are too.
If it were my cat, I'd probably repeat just the CBC in 2-3 weeks and see how the values are this time. If they declined, I'd probably have more tests done.. if they stayed the same or went up, I'd probably just assume my cat is just getting old and has a low blood count.