If you have good flexibility, your age is not a problem at all.
For rhythmic gymnastics coarse, you can join the GAHK's courses.
[GAHK-- The Gymnastics Association of Hong Kong, China]
[中 國香 港 體 操 總 會]
For the coarse which is orgainised by the association and the Leisure and Cultural Services Department, it is much cheaper. Here is the link:
http://www.gahk.org.hk/doc/developscheme200809_ph1.pdf
For the coarse that is organised by the association itself, it is more expensive but the quality is better. Here is the link:
http://www.gahk.org.hk/right02.htm