This took me a long time to find.
According to the USA Track & Field rule book for 2013, there is no maximum age limit for the main "OPEN" and "MASTERS" classifications of race walking. There is however a maximum age limit for other classifications.
Classifications
OPEN
- Minimum age: 14 years old.
- Maximum age: NONE
Junior
- Minimum age: 14 years old.
- Maximum age: 20 years old.
Youth
- Minimum age: 14 years old.
- Maximum age: 19 years old.
Masters
- Minimum age: 35 years old.
- Maximum age: NONE
Please read below.
Except as otherwise provided herein or in USATF's Operating
Regulations, the following championship classifications shall be open
to all member athletes of USATF who have complied with the
requirements of these Rules, the Bylaws, and the Operating
Regulations:
(a) Open: The Open classification shall have no maximum age limit. In
men's and women's long distance running, the Open classification shall
be limited to athletes sixteen (16) years and older on the day of the
competition. In Open men's race walking and all other Open women's
competition, athletes shall be fourteen (14) years or older on the
day of the competition.
(b) Junior: The Junior classification shall be limited to athletes who
are fourteen (14) years or older on the day of competition and will
not become twenty (20) years of age duringthe year of the competition.
(c) Youth: The Youth classification shall be limited to athletes who
have not attained nineteen (19) years of age as of the final day of
the National Junior Olympic Track and Field meet of the year of
competition.
(d) Masters: In Track and Field and Race Walking, the Masters
classification shall be limited to athletes who are at least
thirty-five (35) years old on the first day of the meet. In Long
Distance Running, the Masters classification shall be limited to
athletes who are at least forty (40) years old on the first day of the
meet.