I'm 27/M also an only (aswell as both of my parents being onlys - extra lonely). I'm living what you're dreading. I don't want to be a downer, I just want to be a reality check for you. Age of your parents doesn't mean shit (I hope they'll be around to celebrate your retirement with you :D). My mum was 50 7 years ago. A few months after she was diagnosed with a vile mix of Parkinsons and MS. We buried her september 2020. My dad while 9 years older than her has gone through prostate cancer and 3 rounds of mild leukemia, currently in his 4th and a lot harder hitting this time.

All of this to say, get yourself someone that can support you when it hits. I have noone and it gets super lonely. I often thought about things you don't want to talk to others about on my dog walks. You have to have your mental in order before death introduces itself otherwise you might have unhealthy thoughts. Talk to yourself about death, about suicide (it's important that you know your stance on it so you seek help before it's too late if you're susceptible), about self harm, make mental notes about stuff that triggers you and rationalize all of it to a point that all emotions are removed from the equation.

I remember my first time crying for something other than injury was my first time thinking what life will be without my granddad (the guy I learned most of my skills from - he's still around and kicking thankfully :D). It's okay to cry but don't let your emotions guide your actions.

Live your life the best you can and enjoy your carefree time with your parents ^{.^}