1) Pocket probs
The most tangible of adult fears is money. This is not fear of the paper itself, but the fear of being tight or low on money. Fiscal responsibilities are one of the biggest responsibilities of adulthood. One must be able to manage his budget efficiently in order to be well off and have enough to both cover bills, care for kids (if you have them) and "treat yoself" from time to time.
The remainder of these fears are not tangible things. The older you get, the less tangible things are able to make you happy.
2) Decisions, decisions
Throughout adulthood, there are many big decisions to be made. Choosing a career path, a life partner, making informed health decisions, and deciding things like where to live and work are major choices that can impact your life, be it positively or negatively. Most adults fear making the wrong decision because one wrong decision can steer your life in a certain direction.
There are also many what-ifs behind this fear. For some adults, regret of decisions already made impact their lives as they ponder how things may have been different.
Love is a biggie. Some people are fortunate enough to find love early on and are able to enjoy being with a partner at the height of their youth. Others are not as lucky or always find themselves in the wrong relationships. There comes a point in adult life when love starts to become a huge risk and fear. You feel you are too old for games and do not have time to get played and get hurt, but at the same time, you feel that if you do not find love quickly, you may never find it.
Loneliness goes along with love because no adult wants to spend the rest of their life alone.
As a wise man once said: "YOLO (You only live once)." We only have one life to live that we are all sure of, and there is no telling what may happen. Some adults fear never accomplishing their goals and dreams in life or not getting things crossed off their bucket list before they actually hit the bucket.
5) Losing people
No one is getting younger, so the fear of losing loved ones becomes a thing. For parents and spouses, they do not want to bury a child or their spouse. They also do not want to lose their parents, who may be getting up in age.
Since adulthood is a time where you are only connected to a select few friends from college or high school, you also do not want to lose those dear friendships.
Apart from a few cosmetic surgeries, there is nothing anyone can really do to prevent themselves from aging. At some point, your hair will start to grey and your skin will become less durable and start to sag in certain places. Since no one is experienced in aging until they get to that point, it can be a scary thought. Even as a young adult, I fear aging because it is something unknown.
For the not-yet-parents:
You fear you may have kids at the wrong time or you fear that you may have passed the age that is best for having kids. Adults without children or expecting parents may fear how good of a parent they will be. There is a pressure on you that it will be quite a job and you do not want to mess up, especially if your parents set the bar high for you.
For the already-parents:
You fear that your kids will turn out horrible because of you and that people may judge you. You may also fear that you may scar them negatively for life by doing something wrong or making a mistake. For the adults whose kids are already grown and out of the house, you wonder if you did it right and if you passed on positive, good things to your children.