I purchased a box of LEGO bricks from a yard sale. Some of the 1x6 bricks are hollow with 2 small slants in them. Other 4x6 plates are very thin with square holes in the bottom. It also came with windows with half moon shaped holes in the bottom. Lastly white blocks with clear inserts in the bottom. Does anyone know how old these are?
1 x 6 Bricks
According to BrickLink, 1 x 6 bricks like the ones you describe were made between 1955 and 1969.
4 x 6 Plates
According to BrickLink, there were no 4 x 6 plates with square holes (so-called waffle-bottom plates.) There were, however, 4 x 8's made between 1954 and 1971.
White Blocks with Clear Inserts
The white blocks with clear inserts could be wheel holders, made between 1959 and 1992:
Or some kind of light brick:
It sounds as if your bricks are relatively old, probably made sometime in the 60s. If some of them look a little distorted and don't grip very well, they're probably made of Cellulose Acetate rather than ABS, which would fit with the dates.
To provide any degree of accuracy, however, we really need more information, preferably photos. Also, it might help to know what part of the world you're from, as back in the day there were sometimes different sets for different markets.
The white blocks with clear inserts (provided they have LEGO markings, a clear side window and 2 or 3 holes on the other side - I can't tell because you didn't post any pictures) could be 08010 Electric Light bricks, which were produced between 1980 and 1987.