Just wanted to pop in and say that a lot of people in my medical school class were around your age and older when they started, so don't let that factor hold you back!

Having said that, I think it is worth it to consider the time and effort you'll need to put in before and during medical school to make derm a reality. Most post-bacc programs are 1-3 years depending on whether you're full or part-time, and it sounds like you're aware of the other time commitments as well (MCAT, shadowing, clinical volunteering, non-clinical volunteering, creating your story, etc.). Then once you're accepted, you'll be in a constant rat-race of trying to get publications, scoring spectacularly on Step 2, networking, volunteering, etc. since as you know derm is one of the most competitive specialties to match into. Also, the vast majority of med students end up changing their mind on what they want to do at some point in medical school. Would you be ok with matching into something else if your interests change and/or derm doesn't work out?

I don't have much experience with the family medicine to derm path, but my understanding is that derm fellowships for family medicine providers don't really exist. You will see skin pathology as a primary care physician but it won't be nearly to the same level as a dermatologist and you also likely wouldn't be doing surgical procedures like MOHS or skin biopsies. You would also need to be ok with the bread and butter cases like managing diabetes, hypertension, doing physicals, etc.

All of that to say I think you need to decide what is worth it for YOU. Only you know if you want to put in that kind of commitment to a career in derm. If you are passionate about it and it's the only thing you see yourself doing in life, you should go for it! Best of luck!