About the topic, my 34 year old penis does get hard a lot during the day, for little or no reason. I'll be walking around and maybe some subconscious memory or something passes by my head (or in some cases nothing discernible happens) and suddenly Mr. Happy wants to see the world standing up. Sometimes there's a stimuli, maybe a memory of happier times, maybe a fleeting fantasy, maybe i am really happy to see somebody, but sometimes there isn't. I just shrug and manage the situation as best as i can.

When i was on my teens that was indeed cause for mortification. On the teen years you do feel like it's hard 24/7 or close to it, and super horny. So you quickly learned ways to hide it. The most popular is to catch the head with your pants' waistband, not always a good idea because it carries the risk of it popping out (as some redditor poetically described it, "Then the digglet appears"), but in general works well. There's also manually catching it with your hand in your pocket, which has the problem of sometimes stimulating it, so the boner may have difficulties on disappearing. Then many times you don't get a full-on raging hard-on but just a semi, where it's not quite hard but not quite soft. With those you just adjust it's position on the pants. And there's long shirts.

Yes, a boner eventually goes away unless you stimulate it. Not always desirable, of course, because for one thing, for both genders being aroused feels good in itself, for another your body is both begging and demanding of you that you come now or before, and for another if you've had an erection for too long for many people this means the dreaded blue balls (which are easily solved with an orgasm). If you can, you solve it with a delicious orgasm. But yes, you do learn rather quickly that the enormous majority of your boners in life will be wasted. I imagine this'll bring regrets on the unfairness of life in later years when erections don't come cheaply.

At last, i'd like to comment on an odd boner-related thing i've noticed with me. As i've heard (not much really) this is normal (and maybe even common, i suspect), but since i've always found it odd i'd comment on it.

When i have just the right balance of sleepyness, a point somewhere in the range where it's not enough to make me fall asleep but not too little as to be easily ignored, i tend to pop a boner. Dunno if this is related to morning wood or something, no idea. It doesn't matter if there's a lack of stimuli, and in fact given that it's connected to sleepyness it can happen in utterly unexciting settings. I remember it happening in boring classes in college, with a boring old teacher droning about some topic that probably was very interesting to him and only to him.

At the beginning it was puzzling because i knew it had nothing to do with the environment and somehow knew it had something to do with the sleepyness, but i got used to it. I'd be sleep deprived in a boring quiet moment, sitting down and passing time or listening to classes or working or whatever, and suddenly the flag is raised. Nowadays i just shrug, handle it in the least obstructing way possible and continue with my day, safe on the knowledge that soon enough it'll go away

I hope this has been illustrative, young lady, and i wish that you have lots of penises to amuse yourself with in life (unless you're not into that, in which case i wish you pussies instead).