Outdated plumbing or electrical: This is a big issue to look out for in older kitchens (or houses) because outdated electrical, and even plumbing can be dangerous. Outdated electrical could spark a fire when you are not home, or if you are home. And plumbing older than the 1960's carries a high potential to have galvanized steel pipes which can cause corrosion and clogs. If the need arises to change out the electrical and plumbing you may want to consider remodeling the kitchen when everything is already being pulled apart. Plumbing and electrical will be one of your higher contractor fees so consider that if you are switching over to new age materials and finishings.
Your family no longer fits: The biggest sign is going to be the space in your kitchen. If you moved into your home before children, and now have two or three little ones running around it is going to change the available space in the kitchen. You want to make sure you have time for those family dinners and after-school pizza nights. Hopefully, your kitchen has the ability to expand and knock out some walls if need be.
Open concept: Often times, in older homes they have a much more closed off kitchen or dining room. If you have watched any home remodeling shows you know that open concept is the way to go! People love the ability to spend the time cooking food, with the addition of socializing with your family or friends in the living room or dining room. This could also go along with not having enough usable space. An open concept will create more space for everyone to move around freely.
You don't love the colors or style: I get it, not every home you move into is going to be up to date with your personal style. But with every new home purchase, sometimes sacrifices need to be made as far as what you may love, or what you may not love. What you may not love can always be changed and remodeled to your personal preference. You do not want to go into your kitchen every day and think "ick, I need to change this" or "I can't believe why the sellers put these cabinets in their home". This creates an ideal reason to remodel your kitchen. If you plan to sell your home, but the kitchen is from the 1950's or 1960's you may consider a quick spruce up to help with the sale price. Consider just replacing the countertops and cabinets so it gives it that fresh, modern feel that buyers are going to be looking for.
Your drawers are getting messy: Often times, when we do not have enough space our cabinets and drawers, become overflowed with stuff! This happens because everything does not have its own space, causing us to put stuff in random places (like the oven hehe). With the new remodel capability, it offers new storage solutions like tray dividers or rollouts to help give everything a proper home. It would not be worth the time or money to look for storage solutions for older cabinetry because it could eventually fall apart. So if you are considering an organization makeover, think about remodeling all the "not so enjoyable" elements of the kitchen in the process.
There are many reasons why people would want to remodel their kitchen and these are just a few of them. I hope this helps with your decisions process on when to remodel.