The "normal" age at entry seems to be 17 to 18, at least among those who start immediately upon graduation. Although with the gradual adoption of later birthday cut-offs by one state after another in the last 5 to 10 years, we should be seeing more 19 year-old freshmen in a few years. Although not common by any means,I seem to recall quite a few 16 year-old freshmen at many elite colleges. That seems pretty well accepted and nobody fusses. Although I have read about some over the years, 15 and under is still rather rare and might raise questions, particularly if it is a male, who often tends to look obviously younger.