my two oldest started late (grrrrrr another reason why i was so unhappy with my last vet
lol) ... i want to say they were about 8 or 9 months old? thank goodness their heartworm tests came out negative and i got them on - year round - right after
my youngest, bree, was started on heartworm at about 16 weeks...my vet wanted her to get to a solid 2 lbs and start right away