I take 6, 000 g of vitamin c. Apart from occasional mild diarrhea, I have no other obvious side effects. Are there other negatives for me? I'm 75.

You don't. 6, 000 g of anything is 6kg, or just> 13 lbs. You probably mean 6, 000 mg, which would be 6grams. 1 or 2 grams (! -2, 000 mg) should be more than enough.
Mayoclinic.com/ For most people, a small glass of orange juice plus a serving of strawberries, chopped red pepper or broccoli provides enough vitamin c for the day. Any extra vitamin c will simply be flushed out of your body in your urine.For adults, the recommended upper limit for vitamin c is 2, 000 milligrams (mg). Excessive c:diarrhea, nausea, vomiting, heartburn, abdominal cramps, headache, insomnia, kidney stones.
Too much vitamin C. Use of the vitamin in the way you describe is unwarranted & unwise. Vitamin c (ascorbic acid) probably does not significantly alter the occurrence, severity, or duration of colds. However, long term use of it can interfere with absorption of vitamin b12, enhance estrogen supplement blood levels, cause uricosuria (uric acid crystals in urine), & predispose to formation of oxalate kidney stones.
Yes. Mega dose vitamin c can actually be dangerous. It is true that vitamin c is water soluble & you will excrete any vitamin c that you don't use. However, in order to excrete vitamin c in your urine, you must also excrete minerals as well. Therefore, excess vitamin c can lead to mineral deficiencies, especially a copper deficiency. Be very cautious in megadosing any vitamin or mineral or amino acid.
Be aware of risks... Look up linus pauling. He routinely took (and advocated) much higher doses. If you get stomach pain, or any sign of bleeding, or need surgery, or fall, you could be at risk for bleeding/blood loss. And at 75, you could of course fall at any time and you certainly shouldn't take anti-inflammatories or even aspirin. But i'll bet you don't get too many colds! diarrhea=too much for u to absorb.