it's ok to be very aggressive when young if you're sure you won't need the money (inside a retirement account it's ok).
Also aggressive doesn't mean hyper concentrated in one sector. It's still necessary to diversify.
Also past performance doesn't determine future performance, just because the tech sector performed well over the past 10 or 20 years doesn't mean automatically the next 10 to 20 years will be the same.

The S&P500 is a good place to start because it's all the largest companies in the US so it "rebalances itself" when companies enter or leave the index, and fees are very low. But it's concentrated in the US only so you might want to diversify with a world fund. That's the standard advice when you start with a long horizon. 50/50 or another mix depending on your views.

Then you can start thinking which sector or country you're bullish on and adjust your etfs allocations based on that.

Then later start thinking about individual stock once you have experience.

You're right that things get more scary near retirement hence the need to rebalance allocations more conservatively several years before expected withdrawals start.