Totally normal:). As cactus age they get corking at the base of the plant and the joints. It protects the cactus along with adding strength. Similar to bark on a tree.

Here‘s corking on my big Opuntia. Sorry the pic is upside down;),

Also it, IIRC, strengthens the stem for holding the weight of the pads above, which can weigh hundreds of pounds.

I do not grow opuntia, so maybe I shouldn't comment.

Here is my question: IIRC, corking starts at the base of the plant. I can't tell from he photo if it does or not. Corking is normal for many (maybe all???) cacti. It is usually relatively slow process. But if brown spots apear in other spots on plant, and suddenly, rather than at the base, it is a problem - caused by pests disease or damage.

Rina,

Corking is seen in many cacti, but also isn't seen in many cacti (unless they've have had a traumatic experience). In the tree-like Opuntias that I recently started cutting down, it was noticeable particularly in the major joints, but not in the singular pads themselves. It had no negative impact on the plant whatsoever.

I’ve seen corking start around areoles. That corking looks pretty standard:).