At 90 do I have weak bones or is it natural?

Not necessarily. Our bone density tends to peak in our thirties and then gradually decline, but there are many factors such as genetics, activity level and diet that determines the rate of decline. So while weak bones are more common in 90 year olds, some 90 year olds have strong bones. The only way to know for sure is to do a bone density dxa test, which can be ordered by your doctor.