There is no age limit to start ACCA qualification. You can start ACCA qualification at any time or at any age in your life.
Please do not confuse the age of 21 with the criteria to enter ACCA qualification.
The fact about 21 years of age was not about limitation but it was a relaxation offered to students under Mature Student Entry Route (MSER) by ACCA body. MSER allowed the students to start ACCA qualification without worrying about previous academic record. Means, that if you are 21 years of age then you can start a professional qualification even if you have no academic background of any kind.
But from January 1, 2011 MSER has been abolished. But now ACCA is offering even better alternative.
Previously you must be 21 years of age to claim that relaxation. But since new suit of qualification i.e. Foundations in Accountancy (FIA) has been introduced now every student of any age can start ACCA by registering under FIA for Diploma in Accounting and Business.
So the students who were planning to start ACCA under MSER, and due to any reason were unable to register under MSER, they have even a better way out now.
Anyways, coming back to question at hand, the bottom line is, there is no age limit of any kind to start ACCA. You can start at any time in your life.