I'm not exactly sure where the point where it's much more risky to do any surgery starts I wouldn't think a year and a half would be there yet, but I bet someone else can say for sure.
Whiskers, there are major health benefits to her being spayed. Mammary tumors are a big one, and something that lots of female rats endure. As long as the risk of anesthesia isn't too high, there's always a benefit to spaying and neutering rats.
At this point in her life, it probably won't benefit her much in regard to less, or no, tumors. But, it will stop any chance of reproductive organ cancers, pyometra, prolapsed uterus, et cetera. Personally, I never consider the age of a rat when thinking about surgery - health is first and foremost. If she's healthy/active, and your vet is extremely rat-savvy and has done spays before, I'd go ahead with it.
I adopted two older girls last year in September; their previous owner said they were around a year and a half old. They were spayed as soon as they came to the rescue, and I adopted them only a couple days after. Well, in February/March of this year, both girls developed tumors (one girl had a tumor on the neck, the other had a tumor on the thigh). The tumors were removed successfully, and neither have had any lumps, bumps, or tumors since then