The assumption of many people is that it is safe for non diabetics because it is considered safe for millions of diabetics. Some disagree with that view. During my research, I haven’t found any human studies for the off label use of metformin as an anti-aging drug. There is a funded study which will start soon. Look at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5943638/

Life Extension has recommended metformin as an anti-aging drug since about 1995.  Dr. Sandra Kaufmann, MD, recommends it for non-diabetics in her book, The Kaufmann Protocol. David Sinclair (Harvard Antiaging researcher and author of the book Lifespan) who has tested metformin in mice studies, personally uses metformin as a non-diabetic (but it seems he doesn’t take it everyday).  I believe it is safe enough for me so I started taking metformin in October 2019. I bought it over the counter in Asia.

"Is it safe" is not really the correct question to ask.  The better question is "do the benefits outweigh the risks?"  The answer depends on a lot of personal factors.