How old do you have to be to learn the guitar?
I prefer starting children with guitar instruction at age seven. In general, attention span and muscle coordination is usually more developed. However, there are many teachers that start coaching children at age five.
For adults, you can probably never be too old. As long as you are in reasonable health you can experience the joy of playing an instrument; there has been many studies regarding incorporating music learning for physical therapy, emotional well-being, and spiritual meditative purposes.
I believe studies reveal that music can help improve physical health, mental focus, and spiritual well-being for all ages.