There's no requirement for which vested options you must exercise first, regardless of the age of the grant, the time since the options vested, the time remaining until the options expire, or the size of the option spread. You consider these features of your outstanding options as part of a financial plan that fits stock options, and any restricted stock or performance shares, into your goals. Different approaches are discussed in this website's FAQs and articles about financial planning and diversification.
Alert: Do not let vested in-the-money options expire without exercising them. Know the dates your option grants expire and the exercise rules for termination of employment.
