Female, 50. How do I remove light but long hairs under chin?

Various ways. Laser works best for dark hairs since it targets the dark pigment in the root of the hair. Lighter hair is more difficult to remove but with multiple laser sessions is sometimes effective. Often you need to use electrolysis to permanently remove them - find someone with experience, you can also use vaniqua which slows hair growth while it is used regularly.
Electrolysis. Light hairs do not respond well to laser. For lisolated hairs electrolysis offers permanent method of hair removal.