Has any person ever actually planned his or her consumption according to Modigliani’s LCH?
The life-cycle hypothesis as a model doesn't need to explain people's behavior perfectly to be of use in understanding the economy or the decisions that individuals make. Recall the old saying that "all models are wrong but some are useful." As a quick example, note that many people do in fact borrow when they are young (e.g., for college, save while their income is relatively high (e.g., a lot of people save for retirement), and dissave when they're older---just as would be predicted by this theory.