Can someone explain Optimal Rank with equipment?

Lets say I'm a Con Rank 11 and Physical Level 50. Could I equip Con gear that was lv 50?

Yes, you can equip it, but it will not provide stats that are any higher than gear that is appropriate for your rank. The scaling isn't crippling, it will still be better than wearing nothing or wearing gear that doesn't favor CON, but you'd get better performance out of gear of the right rank. Physical level has absolutely zero effect on gear scaling.

Based on my experience, the scaling is far worse on main hand or off hand equipment than on armor. You can get by with higher-ranked armor without too much of a handicap in some cases, but you will notice a significant decrease in performance using main or off hand equipment even just one rank higher than your current rank almost every time.

Another big issue with gear above your rank is it will degrade much faster. Since higher-ranked equipment costs more to repair, you're wasting gil and losing opportunities for SP by having to do more expensive repairs more frequently.

Robert's right.

The "Optimal rank" is the first job level at which you can equip something without the stat benefits being decreased. "Optimal rank" is determined by your job rank, instead of your physical level.

It's very rare that using a tool or weapon that's got an optimal rank above you will give you better stats than using one with an optimal rank below you. You almost never want to use a tool or weapon that's above your rank. However, you can sometimes get away with using armor that's slightly above your rank.

I believe you can wear lower ranked gear without any loss in stats as well, its only higher that creates a loss?

Yes, MiNuN's right. As long as you are at or above the optimal rank, you should get the full benefit of all the stats.