How do I get a USTA Ranking?
This is info from the USTA.
How to Obtain an Adult or Senior National Ranking
The USTA ranks players in five year age increments from 30 through 90 for men and women, singles and doubles. Any player who meets the ranking requirement described below shall be listed on the Adult/Senior web site and shall receive a USTA ranking certificate for the divisions(s) in which the player is ranked.
Any player who is a USTA member and meets the age requirements can enter a national tournament. A player need not be ranked in his or her section to enter a national tournament. A player who reaches the age of the division by December 31st of that year is eligible.