All FIDE events are organised in two age categories, based on your age at 31 December in the year concerned.
For tournaments in 2021 the qualifying dates for the two FIDE age groups will be 31 December 1971 (for the 50+ Open and Women's tournaments) and 31 December 1956 for the 65+ championships.
There will be no FIDE World Senior Team Championships this year but the individual championships may go ahead in November. There is no information yet about European Senior events which were to be hosted by Poland but we are doubtful they will be held.
The earliest continental Senior tournament that may be held is in late May at Bad Neuenahr in the Rhineland.
Some senior tournaments (especially in Austria and Germany) are still run on the 60+ age group rule (sometimes 45+ and 55+ for women).