Marius’ timeline is less clear than Valjean’s or Cosette’s. In mid-1831, when his neighbors are about to be evicted, he’s 20 years old, and has spent three year climbing out of poverty (after his grandfather kicks him out), studying law, and looking for Thénardier (not realizing that they are his neighbors.) Six months later, in February 1832, he’s been impoverished for five years. I guess he’s rounding up?
Anyway, he would have been born in 1811, while Napoleon was still in power. Looking back at an earlier chapter, his mother died in 1815, though it’s not said whether before or after Waterloo. That means Marius was already four years old when his grandfather took him, so he ought to have had at least some faint memories of his father, though perhaps he was away at war for most of that time.
The other interesting thing about this is that Marius meets Enjolras and company at the beginning of those three years, then stops hanging out with them (except for Courfeyrac, the one he’s actually friends with) after he embarrasses himself by rambling about how great he thought Napoleon was. That spans the July Revolution of 1830, meaning the ABC Society is already active at the time of the revolution…but we don’t see anything about their involvement!
When I mentioned this on Twitter, Prof. Lewis of the Les Misérables Reading Companion podcast replied, suggesting it was because they would have considered the July Revolution a failure, and as such it wouldn’t have had much impact on their goals or activity.