Without a photo or personal consultation, it isn't possible to offer advice about whether you would be a good candidate for a facelift. I can say that there are some people who can benefit from a facelift at 51. Age isn't the primary factor; rather, it's the extent of sagging tissue. I suggest consulting with one or more board-certified plastic surgeons to discuss your specific concerns. It's possible that nonsurgical treatments may be the best path for now, but you must consider the ongoing maintenance costs versus the one-time price of surgery.
There's really no ideal age to have a facelift. Some patients would rather have the surgery at a younger age so they have many years to enjoy the results. Others really want a more dramatic change and wait until their 60's or even sometimes their 70's. It really depends on the individual and the most important thing would be to seek consultation with a very reputable facial rejuvenation specialist who can give you an honest opinion.
Facelift surgery is most effective for treating saggy jowls and loose skin especially in the chin and neck region. These are common occurrences with the aging process. These changes also do not occur at any particular age. I have performed first facelifts on patients in their 30s and others don't require a facelift until they are in their 60s. If you are looking in a mirror and using your hands to pull your jowls up back towards your ears, you may be a candidate. An experienced surgeon will inform you about all aspects of the surgery including expected results and also how long you as an individual would be expected to keep those results.