Could you please provide a detailed explanation of the age limit requirements for the RRB exams, including the specific age range within which candidates must fall in order to be eligible to apply? It would be helpful if you could elaborate on any variations in the age limit for different categories of candidates, such as those in the general, OBC, SC, ST, and other reserved categories. Additionally, could you clarify if there are any age relaxations provided by the Railway Recruitment Board for certain groups, and if so, what the exact rules and conditions are for those relaxations? Furthermore, please explain how the age limit is calculated, such as the reference date or cutoff date for determining the candidate's age, and any other relevant details that candidates should be aware of regarding age eligibility for the exam.