The third stanza draws an analogy between money (or one's personal relationship to money) and life itself. According to the speaker, it's pointless to try to "save" either.
First, in line 9, the speaker says that life and money have "a lot in common." Then, as an example, line 10 points out that "You can't put off being young until you retire." In order to get the maximum satisfaction from youth, "You" have to spend it quickly, proactively, even impulsively—much like money, it's implied. (Ironically, it's often retirees who would have the time to enjoy youth, if they were still young. Meanwhile, young people are often too busy working.)
The speaker expands this analogy in lines 11 and 12. However people save their money—or, in the speaker's words, "bank [their] screw"—the money, "in the end," won't buy "more than a shave." Larkin could be thinking partly of inflation, which was rampant when he wrote the poem, and which devalues money over time as the price of goods and services rises. But he's mostly making a point about mortality: people can't enjoy spending their money once they're dead! Both youth and money go to waste if people sit on them too long.
More broadly, the speaker feels that both life and money are, in a sense, futile. People can work hard and save for a long time, but it all adds up to nothing—you can't reverse the spell of aging, nor prevent death.