With people that I know, there were many who the older they were, the less side effects they had.
ご質問ありがとうございます。
"With people that I know"=「私が知っている人達の場合」
"there were many who"=「~である人が多かった」
"the older they were"=「年が上がれば上がるほど」
"the less side effects they had."=「副作用が少なかった。」
("had"の代わりに"showed"=「見せた」も使えます)
ご参考になると幸いです。