FIFA has finally published the circular with guidelines to be followed by national federations, for the nominations of FIFA referees since 01/01/2019.
The deadline is 01/10/2018. The proposed referees shall be at least 25 years old (23 years old in the case of assistant referees) on 1 January of the year for which they have been nominated. Referees must be born no later than 31.12.1993 and assistant referees must be born no later than 31.12.1995 in order to ensure a minimum level of experience. FIFA reserves the right to require referees over the age of 45 (born in 1974 or before) to undergo additional technical assessments as well as specific medical examinations and fitness testing on a case-by-case basis.
2019 FIFA Refereeing International Lists - PDF