Yes, Brexit notwithstanding, we've had a nice solid economic boom for the last 9-10 years, so those numbers are not averaged across any recessions. *That may be why those ETFs were started then - it's possible they folded up other ETFs so they could start new ones with fantastic numbers like this.
What to expect: the nonprofit view
A nonprofit (not-for-dividend) "endowment" is when someone donates a large chunk of capital, and it's to be invested forever, sustainably to keep up with inflation. The excess is to be drawn down in a prudent manner, and it funds various programs - such as a named "chair" at a university. There is strict law as to how endowments must be invested. They are not allowed to sit idle in money market funds or bonds - they must be in the market at least 60-80%.
Prudence is the key. If you have a 35% market upturn in a year, you are not allowed to spend the whole 35% - you must put those acorns away for a future downturn! On the other hand, in a downturn, you're still allowed to spend the normal amount of money. (Super important if your mission is to help the indigent!!) All in all, 4-7% a year drawdown is presumed to be "prudent" - anything else, you'll need to explain yourself! That is my experience as an endowment manager through the last recession.
So as a rule, I expect index funds to yield 7% + inflation over the very long term.
The real place value is found
I get you're looking at past returns while looking at funds. That's not quite what to look for. The #1 risk in an index fund is the guaranteed losses of the expense ratios and loads - the entry, exit and annual/continuous fees they charge you to be in the investment.
As far as comparing returns, it doesn't help to compare them absolutely -- and you're right, ones that started 9 years ago will compare very favorably to ones that have existed for longer and ridden through recessions. That means nothing, as you surmised.
What matters is the performace of the fund relative to the performance of the index it seeks to track. If an S&P 500 fund loses 1.5% a year compared to the S&P 500, it's got a problem. Where do those problems come from? Internal fund expenses, which are guaranteed loss for the investor. As such, keep them minimized.
- Expense ratio, or the management costs they charge you to have your money in the fund. A common managed fund (and many index funds!) charge 1.5% per year, which pays the "genius stock picker" and his staff. They are marketed to "sucker" customers who let their broker pick their investments. Index funds can (don't necessarily do) reduce expense ratio dramatically, because an index fund is simply synchronised with the index, and an intern can do that. Vanguard index funds charge as little as 0.04% expense ratio.
- Sales loads - such as front-end loads, which can be 5.5% of the investment. Invest $10,000, the very next day you have $9450. Seriously. This goes to sales commission for the broker. There are other loads, such as back-end loads, payable when you sell. Some funds are no-load. You should buy only those.
With ETFs, the expense ratio is built in to the trading value of the fund. In a perfect world, an ETF will underperform the index by exactly their expense ratio.
So when selecting funds, start at "loads" and "expense ratio". Then compare their performance to the performance of the index.