Different people experience different symptoms, but the usual ones are: unexplained weight loss, fatigue, frequent thirst, frequent urination. Some of these can be attributed to age, which makes diagnosis more complicated. With me (11 years ago) it was mainly weight loss & fatigue, plus family history. Sister has been diabetic for 35 years, brother diabetic, mother mildly diabetic.
As for living just fine with higher blood sugar, it depends on who you ask. Most doctors will say even slightly higher count should be treated aggressively. I think they are wrong & are simply repeating what they've been taught. Current research says treating older people with elevated blood sugar is too risky due to higher risk of hypoglycemia - low blood sugar, which can cause falls, coma & death & is much more dangerous than high blood sugar. In fact, doctors are now required to tell their patients "Low blood sugar is far more dangerous than high blood sugar."
My blood sugar is high most of the time & treating it with insulin causes lows a couple of times per week. I won't take oral diabetes medications because I'm not convinced of their safety. It's rather funny - those drug ads that say, "This drug has caused amputations, heart attacks & strokes;" exactly the problems we're told are caused by diabetes. When these drugs cause problems, the doctor will say, "You're diabetic; what do you expect?" instead of blaming the drugs.
When I see my doctor (not for advice, but only because she won't refill my prescription for insulin without seeing me), she pushes the latest, newest insulin & drugs, even though I'm doing fine with my current insulin. Coincidentally, as I was checking out, a drug company rep. with a name tag "Merck" was also leaving. Well doctors get great perks for prescribing new drugs; that's what motivates them to suggest changing medications....it has nothing to do with patient care.