I get those ugly marks on my forearms periodically, usually the result of a fresh mosquito bite that was extremely itchy so I scratched my skin too vigorously. (There are lots of mosquitos year-round in rural Hawai’i and I spend a lot of time outside, so it’s an ongoing situation.) They are unsightly but otherwise harmless.
I can attest to this. Seems I started noticing them somewhere in my late 50s. I’ve also noticed how much easier it is to remove skin by bumping against minor things, or when peeling off a bandage. Ouch!
If you are interested in the nitty details of why …
Worse as a consequence of greater sun damage.
Not to make light of your situation, but now that our thread’s real doc has said it’s nothing too terribly unusual or dangerous. When I read this …
I immediately saw the Black Knight with a big spurt of blood arcing upwards from one of his severed stumps. Got an actual laugh out of me it did. Carry on.
A tip for anyone who hasn’t come up with it on their own - if you have problems with getting marks from bandages, and you use a lot of bandages because those bumps and scrapes bleed, try using sterile pads and self-adhesive medical tape (one of a dozen examples) instead.
Mr. Legend gets these, and has since about the age of 50 or 55. His mother used to get them, too. My mother never got them, nor do I, even though we were both more fair-skinned and bruise very easily. The difference is that my mom and I both made extreme efforts to stay out of the sun because we both burned so easily, while Mr. L and his mom loved being out in the sun.
You say that they occur out of nowhere, but given how easily they form, you probably just don’t notice. He used to come back from business trips with them up and down both forearms; it took him a long time to realize that if the strap of his carry-on bag slid down his arm even a little, it would leave a giant mark.
Now that we’re talking about it, I remember that Mr. brown said that these bruises can suddenly bleed if he absent-mindedly scratches at them, not that they itch. And he works at various things around the house and on the cars and scrapes and bangs his arms slightly all the time. They’re the kind of impacts that don’t even register because they’re so slight, but they’re enough to cause these bruises.
My previous boss starting getting them in her late 60’s. It really pissed her off. Neither of my parents had them, so I’m keeping my fingers crossed (gently).
My mom ended up switching to all white sheets on the bed so she could bleach them, as dad would bleed all over them. She probably should have gone with maroon but it clashed with the room paint 
If Dad had bled enough she could have just repainted the room with …
No, that’s not nice. 
When I was a kid I often helped my Granddad doing chores on his farm. Quite frequently he would bleed from brushing up against something but be unaware of the fact that blood was running down his arm or hand, and I couldn’t for the life of me understand how anyone could be actively bleeding and not know it!
Now, in my 60’s, it happens to me all the time. When I’m out fishing, or just hiking in the woods and drag a twig across my arm, I bleed. Reach into my toolbox and rub the back of my hand against the edge of the lid, I bleed. Bump my arm against the edge of a table or countertop while doing something, I get a huge bruise that looks like someone hit me with a baseball bat. Sometimes I know when it happens, but other times not until the blood makes my fingers wet or sticky or I see it drip on what I’m working on.
I’ve always attributed it to the fact that I spent a lot of time over the course of my life working out in the sun before wearing sunscreen became a thing and that UV damage over time is what made my (and my Granddad’s) skin so papery. It’s nice to know that it’s really just an aging thing for a certain percentage of the population!
Well damn, another thing to dread in 15 or 20 years. As it is this already happens during particularly cold winters: the minor-est of scrapes against my car while trying to clear it of ice leads to a bloody cut.
I have mild dermatitis, which sometimes gets bad enough that I scratch and bleed in my sleep. My experience is that a pre-treatment like Shout works fine to get bloodstains out of sheets of any color.
Age spots should not bleed, they are basically large freckles. Skin spots like this CAN be a sign of liver problems. Had your blood work checked lately? If not it may be time. You don’t take any Tylenol type pain killers on a regular basis, do you? Because that shit will kill your liver.
Do not ignore this, see your doctor and have them tell you to ignore this.
I did do this. It’s in the OP
Both my father and grandfather experienced this in older age. I expect I will as well.
Another thing that comes the closer you get to death.
My mom had them all the time. Especially on her arms, and around her ankles. She also needed special bandages when medical stuff was done to prevent her from losing skin when the bandage was removed.