Not contributing later in life
It's true that as you get older, any investment contributions will benefit less from compounding than would contributions you made earlier in your investment career.
In a related vein, IRA contributions made later in life will benefit less from tax-free (Roth) or tax-deferred (Traditional IRA) compounding than will contributions made earlier in life.
That's not to say you should forget IRA contributions in the years leading up to and during retirement, however. For one thing, you may have 20 years or more of tax-advantaged compounding left.
And if you're investing in a Roth IRA, you won't need to take distributions from your account unless you need the money (that is, there are no required minimum distributions), so your money could continue to compound even after you're retired.