An example of a good 80th birthday toast is, "Let's mark the milestone by celebrating the grand life of a grand person." Toasts for an 80th birthday typically focus on the milestones and accomplishments of such a long life.
A personalized toast for an octogenarian is always appropriate. For instance, if the celebrant is a grandmother, her grandchildren can make a list of how she has added to their lives and incorporate them into a toast. So, if she is an excellent baker and has offered life advice, the toast can start, "From your delicious brownies to your time-tested advice."
Another option is simply to remark on how valuable the octogenarian has been. For instance, someone giving a toast can remark that the gift given for the birthday has a price tag, but the gift of wisdom from the celebrant is priceless. Similarly, the toast can include sentiments stating the 80-year-old is a "walking, talking treasure of life experiences and perseverance."
A birthday toast can also be mildly humorous. For instance, the person can say that the octogenarian carries a high price tag for being "an antique." Likewise, the toast can include the observation that life has come full circle, allowing the 80-year-old to behave like a child again.