As you may or may not know, I live with two types of invisible disabilities. The most common one that people are aware of is cancer. The less spoken affliction is in my back and shoulder.
Long ago, before cancer was handed out to me, there was an accident. There was a chain reaction created by someone else that left me injured. It's not an injury that's easy to monitor/explain, it is nerve damage. I now live in constant pain. In the morning I wake up with pain, in the afternoon I have worse pain, and in the evening I have even worse pain.
They haven't found a cure for it, the medical field has found some ways to help, slightly. I get nerve blocker injections monthly. These help make the pain more of a toddler's fury than a rabid badger being fed steroids. They only work temporarily, and the pain level goes back to being as intense as ever.
I'm not writing this for pity. I want you to understand something. Not all disabilities are created equal. I can walk, I'll even make it look like it's not causing me pain, but I've had times that my body almost fails me and I need to sit down quick or I'll fall flat on my face, from pain. Imagine that. You're walking and suddenly your body says, "No more". Can you easily live with that looming overhead? I have a disability permit because of this. I might start out okay when I get to the store, but line ups, avoiding crowds, even just walking the aisles might trigger my nerves to worsen beyond my capability to stand it. I need my vehicle to be nearby. Others have reasons that are invisible, too. Common ones are crohns and cancer.
Don't glare at someone younger getting out of a vehicle with a disabled permit. They have the permit for a reason. Be responsible for yourself. If you're not disabled, don't park in the spots for them. If you're not pregnant or with infants, don't use their spots, either. If you're not a senior, leave their parking alone. It's not that difficult. I know you're in a hurry, and you'll only be a minute, but a minute when you feel like you're being stabbed in the back with razor sharp daggers is an eternity.
Everyone is fighting their own battles, don't add to the attack.