1. A contract of service/appointment letters normally contain basis terms and conditions of service whereas a company handbook contains other rules and regulations, company policies and other benefits which are not mentioned in the contract of service. Normally, the contract of service has a clause making reference to the handbook and some companies even attach a copy of the company handbook.
2. It is true initially, the company did not have a company handbook. When they decided to have one, the terms and conditions in the handbook might turn out to be different or less favourable the those in the contract of service. As you said, in the contract of service, there was no mention of retirement age and suddenly there was one in the handbook. A retirement age may put the employee at a disadvantage because he or she is not entitled to termination and layoff benefits when retired as mentioned in Regulation 4 of the Employment(Termination and Layoff Benefits) Regulations. So, employees must be alert. They should read carefully what were stated in the handbook. They should bring the matter up when they find somethings not in their interests. For example retirement age. While they are still young, they may not be bothered about retirement age of 55 in the new handbook. But when they reached the retirement age and asked to retired, then began to realise and ask "which document should one refer to".
3. So, it is quite difficult to say which one one should refer to when handbook was not mentioned in the contract of service. If the handbook had been there for years and no one ever protested. That could be interpreted as consent. However, the final decision on this lies with the authority based on facts. You can always consult the Industrial Relations Department for an official opinion.