My DD12 (in seventh grade) took Algebra I over the summer and is now in geometry. It was a lot of work because she simultaneously took AoPS for rigor and an (IMO) inferior course that is taught by Washington-certified teachers. But it wasn't by any means too hard - she did very well in both courses. (It took a little convincing to get her to believe that she did well in AoPS, because she actually had to work on the problems, and sometimes gave several wrong answers before she found the right one. I had to give her a number of pep talks about how that is how math is supposed to be.)
I don't see any reason why an 11-year-old seventh grader shouldn't be able to succeed in Algebra I.
