Can carbon-14 be used for objects older than 50 million years?

No. Half-life of carbon-14 is nearly 500 years. For that reason its limit is just 50 thousand years. As far as I know, dating of 100 milion year old "something" is hard. Because you can always have some (maybe 1-3 milion years) uncertainity. For dating that object you can use potassium -argon, uranium-238, thermoluminesce and electron spin resonance.