There is no age limit to apply for Federal Disability. If you are currently employed or are within 1 year of separation, then you can apply for Federal Disability Retirement. However, if you are approved and already meet the regular retirement eligibility, then your annuity will be calculated at the regular retirement rate instead of the disability retirement rate.
It can be beneficial to apply even if you already meet or will soon meet the eligibility requirements for early retirement if you are entitled to a large amount of backpay.
Other factors such as age and service requirements can also play into your eligibility.