There is nothing that young people can teach older people.

Claiming that there is nothing that young people can teach older people is a very courageous option. One can rely on the fact that older people have lived longer more, hence they have more life experience, while younger people are only beginning start to learn from these experiences. However, I believe that accumulated life experience have has nothing to do with age of human, and young people always have something to share with older ones as well much as the olders have. And even more, Also, as far as I am concerned, younger people can help older people to adapt in the world of rapid technological change, and they're usually considered as being more creative and resourceful.
As I said, younger people can help older people to adapt in the new world. For example, we had a problem with my lovely grandma, because she couldn't use a cellphone. With At the start of September she forced to ( or had to ?? ) stay at home alone because my mom went to work and I had a classes. We were afraid that something bad would happen and she  would not be able to inform us. That was difficult but we taught teach her to use the basic commands of her cellphone and called as at least twice a day.
Thanks

There is nothing that young people can teach older people.

Claiming that there is nothing that young people can teach older people is a very courageous opinion. One can rely on fact that older people have lived more, hence they have more life experience while younger people have only started to learn these experiences. However I believe that accumulated life experience has nothing to do with the age of people (age of humans is more to do with the history of people and not their actual age)

, and young people always have something to share with older people and vise versa. Furthermore, as far as I am concerned, younger people can help older people to adapt in the world of rapid technological change, as they are usually considered to be more creative and resourceful.
As I said, younger people can help older people to adapt in the new world. For example we had a problem with my lovely grandma because she couldn't use a cellphone. With the start of September

(From the beginning of September sounds more natural but with the start of September is fine but it is something you might find more in a book than everyday speach)

she was forced to ( or had to ?? ) (was forced to means someone made her but had to meant she did it more of own choice so her forced is better)

stay at home alone because my mom went to work and I had classes. We were afraid that something bad could happen and she couldn't inform us. That was difficult but we taught her how to use the basic commands of a cellphone so she called us at least twice a day.
Thanks

Well done.  I hope my corrections and explanations helped you.