New York (CNN Business) Stocks were up Monday on the news of a US-China trade truce. But it may be tough for this aging bull market to keep powering ahead for much longer -- even if China and the US reach an actual agreement.

Earnings growth north of 20% for the S&P 500 has helped power the market in 2018. But profit growth is expected to slow to less than 10% in 2019.

The series of interest rate hikes by the Federal Reserve could begin to slow down the economy as well, especially as the stimulative impact from tax cuts begins to fade. That's why the big jump in profits this year may be unsustainable, according to JPM Asset Management chief global strategist David Kelly.

"With both wage growth and interest rates expected to rise further next year, margins should begin to come under pressure," Kelly and his team wrote in a recent report.

Trade could also hurt stocks next year. US President Donald Trump and Chinese president Xi Jinping are not guaranteed to reach a deal before higher tariffs go into effect. And it's not as if the United States is only fighting a trade war with China: It's also involved in several trade spats with other nations. The impact of all this trade tension could lead to more volatility.

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