Are some people that are inflexible, never able to be flexible, even through practice?

You can increase your flexibility to a certain extent through training, but some people are naturally more flexible than others. So to answer your question, as an adult even with lots of training normally your flexibility will be limited in how it increases. Men are almost always tighter than women in the hamstrings/legs - it's a physical difference. A good way to increase flexibility in hamstrings is to stretch out your lower back. Tightness in the legs is often due to a stiff lower back.