Yes she is old enough for most parasite control products. Personally, I like the fipronyl products (Frontline and many generics) as they provide good flea and tick protection. It is okay but not necessary to use the entire tube, for 5 lbs i would use about 1/3 tube or let me know what product you select and i can help with dosing. Check with your vet as there are geographic, environmental variations that influence the choices. If you haven't seen the "Flea" and "Parasite Control" videos you might take a look.