There's an ongoing debate about whether it's better to give now or later. A quick summary:

Reasons to give now:

You may get less altruistic as you age, so if you wait you may never actually donate.

Estimates of the returns on investment may be over-optimistic.

Giving to charities that can demonstrate their effectiveness provides an incentive for charities to get better at demonstrating that they're effective. We can't just wait for charities to improve — it takes donations to make that happen.

Having an active culture of giving encourages other people to give, too.

Better to eliminate problems as soon as possible. E.g. if we had eliminated smallpox in 1967 instead of 1977, many people would have been spared.

Reasons to give later:

As time passes, we'll probably have better information about which interventions work best. Even in a few years, we may know a lot more than we do now and be able to give to better causes.

Investing money may yield more money to eventually donate.

When you're young, you should invest in developing yourself and your career, which will let you help more later.

You can put donations in a donor-advised fund to ensure they will someday be given, even if you haven't yet figured out where you want them to go.

But it’s a topic that deserves more depth than that summary. Here's some of what's been written on the topic, in roughly chronological order: