You canít assume that one size fits all when it comes to the seat height of chairs and benches, but most are designed that way.
Typically, seat height is kept at about 18 inches. However, in some applications (a dining chair) thatís a bit high. So, you may find some dining chair seats at roughly 16 inches high.
Iíve designed them with sizes all over the place, mostly because of the look of the project, but partly because of the function. If the project is meant to be used pretty much anywhere, a utility chair or bench, then I may set the seat height at the standard 18 inches. An outdoor bench may get a little lower, even as low as 15 inches. Why? A lower seat seems a bit more comfortable in an outdoor setting. Plus, an outdoor bench is used by a wide range of folks including kids, and a lower seat is easier for them.
Which brings me to another point: Itís rare that I design seating specifically for kids. Youíll see a childís stool project here and there (I consider stools in a separate category from other seating) which will be a bit shorter. I do this because I look at furniture for children as being temporary, unless a piece is a specialty project passed along to family members. I made a rocker for my nieces that I consider a specialty piece, for example.
My point? Have a little flexibility when designing your own projects, or building from a set of plans. If you need a higher seat (folks with joint issues find a higher seat more comfortable), adjust the plans Ė not so much that you drastically change the balance of a project Ė to meet your needs. If you do make a seat much taller, be sure to adjust the depth (and width if necessary) to be in line proportionally. If you want the project to be more universal, drop the seat height down a bit.