Is it permissible for a 64-year-old widowed mother to travel to perform Umrah without a mahram, with tourism companies, provided that her son joins her after one or two weeks?

If it is her first 'Umrah and she has not performed it before, and she finds trustworthy companions, then there is no harm in her traveling with them according to many scholars. However, if she does not find trustworthy companions, then she is not permitted to travel with them without a mahram.