
There was a limited study many years ago that indicated that if a bitch was spayed before having a season her chances of having mammary tumours was reduced by around 80% I think it was. That if the spaying was done before the third season the benefit was still around 50% but spaying later than this had no effect at all.
So spaying at 7 years old will not help reduce the risk of mammary tumours.
As a breeder I obviously do not have my bitches spayed until middle age, and of the 4 mature bitches I have aged 3 to 11 years only the oldest had a tiny mammary tumour at 5, (was mot benign) which regrew at 8, and has been no problem for lat 3 years. On the first occassion the tiny lump was removed, and on the second the whole gland. I have twice had two fatty lumps removed elsewhere on the body (benign Lipomas) as I just wanted to be extra careful.
Her 8 year old spayed daughter has shown no signs of any as yet, and the next youngest (entire as yet) at nearly 6 has not had any either (both have been bred from). The youngest is a maiden bitch.
There was some idea that in humans breat cancer was reduced in women that breast fed. Don't think this translates into dogs, as my oldest girl had two litters before having a lump.