…that the grandson of the tenth U.S. President is alive today?

Yup.

John Tyler was the tenth president of the United States, serving in office from 1841 to 1845. In 1853, at age 63, he fathered a son named Lyon Tyler. At age 75, Lyon Tyler fathered Harrison Tyler, who was born in 1928. Therefore, the grandson of a US president born in 1790 is still alive today.