But this does not matter, for, as he has often remarked, one is never too old to learn

这句话里as he has often remarked是插入语，补充说明，所以as he has often remarked前后都要加逗号隔开。

原话可以改为：But this does not matter, for one is never too old to learn。

但这没关系，因为正如他常说的那样，活到老学到老。

for在这里用作连词。是用来解释前半句话的意思。
整句话翻译是：但是这并不重要，因为（for的含义）正如他所说的，一个人不会因为年老而不去学习。