It's not that bad.
First of all, distinguish between hypothermia and frostbite. The latter is when your flesh actually freezes. Water crystals do a lot of damage. It is extremely painful when thawing.
If you get cold enough for numbness to set in, that too will be painful on warming.
Hypothermia, not so much.
First symptoms: The 'umbles: Stumble, Fumble, Mumble. Your fine motor coordination goes. Note: Mumbling may be just caused by cold cheeks and lips. Mumbling in this case is much more like someone who has had too much booze.
Involuntary shivering. If you can force yourself to stop with an act of will, you are still ok. In danger, but ok.
Sometime in here you stop being rational.
Once you stop shivering you are in big trouble. Muscles are too cold to work, which means they have stopped generating much heat. Your body temp drops really fast now.
As your core cools, heartbeat gets irregular, your lungs start filling (pulmonary edema) and death results.
The shivering phase is unpleasant, but not painful.
Note that in my outdoor experience I've seen mild to moderate (uncontrolable shivering) on several occasions.
The symptoms do not always show up in the same order.
Sometimes this may be that no one (including the victim) noticed. Sometimes they just don't happen that way. Some may be the complications of low blood sugar, dehydration, exhaustion.
Note that hypoglycemia (low blood sugar) will also show the 'umbles and the irrationality. Dehydration also has similar onset. Both make hypothermia more likely, the first because there is less fuel to create heat, the second because the circulation system is sluggish and can't move glucose, O2, and heat around as effectively.
In all three cases treatment for early stages: sugar and hot drinks,
For hypothermia: dry/more clothing, shelter from wind, more movement if conscious.
It does NOT have to be below freezing to die from hypothermia. You can die of hypothermia in 70 degree water. Takes hours. In 50 degree water far less. In 35 degree water you aren't able to do much for yourself after about 20 minutes. Unconsciousness is another 20 minutes or so.
For non-water sports, the dangerous temps are around freezing. It's hard to stay dry at those temps. But being immobile on mountain ridge at 50 F in a storm can kill you with hypothermia.
Wind makes it worse. It keeps moving the warm air next to your skin away.
Wet makes it worse. Water evaporates and cools. It also reduces the effectiveness of insulation. The combination is deadly.
Small people (children, women) are more susceptible than larger.
Skinny people are more susceptible than fat people.
In both cases, the core is closer to the surface.
If you get to the cesation of shivering, you have a life threatening situation. If you are far from help the victim is very likely to die.
Don't let it get that far.
Training: Know the symptoms. Teach the symptoms. Teach the responses. There is merit in going out on a cold windy day, having people peal down then put on a soaking wet t-shirt. Do this near a heated building obviously. Talk about the symptoms as they occur. When someone can't stop shivering, go inside. Have hot drinks and a roaring fire handy.
Buddy system. Pair everyone up. Practice calling out "Buddy Check" and have people talk to their buddy. Since 'umbles are usually first this is a quick check.
More experienced personelle double check. If hiking, the easy way to do this, is to step off the trail and look at faces as they go by. Fall in and chat to any zombies. Do this from the start of the trip to recognize the differences.
Keep people hydrated. Lots of people won't drink enough if it's just water with no flavour. Make sure they drink. Train them to notice their pea color. Colourless to light yellow. No problem. Dark yellow to orange. Get them to drink. (Some foods and vitamins will darken your urine.)
Keep people fed.
Every case of hypothermia I've seen has been near the end of the day when people were running on empty. As leader on day hikes, I always carried a couple of bread and honey sandwiches (easy fuel) If I didn't use them, I took them for lunch the next day. Candy bars work well too. Eatmores and snickers are hard to eat when cold. The mini bars available at Halloween are easier to eat, and easy to share.