This year the 401(k) plan hits age 40. The plan has evolved from what it started out as — a relatively simple savings plan intended as a supplement to a pension plan — to become a replacement for pension plans, which are going the way of the dodo. That doesn't mean the structure of the 401(k) has stayed the same. In its 61^st annual survey of profit sharing and 401(k) plans, the Plan Sponsor Council of America has found plenty of changes—and not just in structure, but in features to make such plans more effective savings vehicles. Contributions are up, from both employers and employees, and auto features are helping to make the 401(k) more effective in its mission. Technology is changing the way employees interact with their accounts, and employers are changing plan designs to make them more accessible and easier for employees to use—not to mention incorporating ways to encourage them to save more. In addition, 90 percent of U.S. employees at respondent companies are eligible to participate in their employer's DC plan—and while that's stayed fairly consistent, that still leaves a considerable number of employees without access. About two thirds of companies allow part-timers to participate, but there are still limitations on eligibility; while 60.1 percent of companies allow workers to participate immediately upon hire, the rest do not. And more than half of small plans require employees to be at least 21 years old to participate. The slides above show the 10 key changes that have been incorporated into 401(k) plans over the last 40 years. READ MORE: 401(k) savers increasing their contributions Interactive infographic: Which states have the best 401(k) plans? 9 ways to help 401(k) participants save more