There is a growing divide between those who earn a bachelor’s degree by age 24 and those who don’t, with the gap between the richest and poorest students doubling in the last four decades. The percentage of students from the lowest-income families — those making $34,160 a year or less — earning a bachelor’s degree inched up just three points from 1970 to 2013, rising to 9 percent from 6 percent. College completion for students from the wealthiest families rose dramatically, though, climbing to 77 percent from 44 percent. The widening gap in college completion mirrors a growing divide in income inequality: While pay for the richest 10 percent of the US has jumped in recent decades, for most Americans it has stagnated, after accounting for inflation. The percentage of students from all income levels enrolling in college has increased over the last four decades. There was a 46-point gap between rich and poor in 1970, compared with a 36-point gap in 2012. But completion gaps are growing: While 99 percent of students entering college from the highest-income families — those making $108,650 or more a year — graduate by age 24, just 21 percent of students from the lowest-income families finish by that age. College costs were more than two times higher in 2012 than in 1975, at the start of the Pell grant program, which provides aid to low-income students based on need. Pell grants covered 67 percent of college costs in 1975 but only 27 percent in 2012.