Mo. Rev. Stat. § 86.133

The proportion so computed for a person age fifty-nine shall be applied to a member who attains a greater age before he becomes a member. The board of trustees shall certify to the board of police commissioners and the board of police commissioners shall cause to be deducted from the salary of each member on each and every payroll for each and every pay period, the proportion of the compensation of each member so computed. But the board of police commissioners shall not have any deduction made for annuity purposes from the compensation of a member who elects not to contribute if he has attained age sixty, nor shall any member be required to increase his rate of contribution as a result of any valuation or revision of members' contribution rates.