My instinctive response is 26-36.
I might possibly say 26-39 but a ten year age gap is not small and, as you get older, might hold you back and tie you down. You don't want to be dealing with middle-aged lethargy and grumpiness while you're in the energetic prime of your 30s. You certainly don't want to be nursing an old man when you're just retired and are ready for travel, leisure and adventure.
I kind of think that good men get snapped up early and, while there will be exceptions, the field narrows to a shocking paucity by their 40s. (Good people break up but the men particularly, usually re-form relationships fast. Usually with someone they already knew).
Yet, most young men are immature. It's a conundrum. I think that either you catch one young, when you are too and grow up together. Or, there's a golden moment, when the decent ones go from young and carefree, to grown-up and open to serious relationships.
I've seen that happen in the blink of an eye, literally 'right, I've sown my wild oats / travelled the world / enjoyed establishing my career with no ties, now I'm ready for the serious relationship'. At that point they either settle down with the next girlfriend who comes along (right time, good enough person), or, having played the field, they know exactly how to narrow their search to go after Miss Right.
When I've seen that happen, it was at age 30-36. Turning 30 is often a trigger.