I just love rhubarb! It gives so much and asks so little. If I had an old bed like that, I'd top dress it with some compost, water it in a bit, give it a mulch and pick it this Summer. Then, when it goes dormant in Fall, prepare some new beds (with lots of organic matter and deeply dug, rhubarb sure likes to feed rich and will bear well if well fed). Ideally, give each new clump an area of 5x5 feet. I divide my established clumps into root balls about 18 inches across. (in an old clump, tho, there will be HUGE tap roots, hard to dig up but worth the effort as the deep tap roots make them fairly drought proof). Where you have only one huge patch, you might want to leave part of the patch intact and only divide some of it, so you will have something to pick from next year. That is because it is counterproductive to pick from a new clump the first year (while it establishes itself). I like to divide clumps about every 4 years, they do so much better with fresh soil as they deplete the nutrients in the area after years and a large clump cannot get enough water in dry years. It is hard to enhance to nutrients of established plants, tho they benefit from a good top dress of compost in the Fall and a good mulching. Over the years, this process will keep you in a vigorous harvest and build your supply (and, when you have all the plants you want, make a great product for swaps or giving "starts" to friends!).
Rhubarb doesn't get "ripe". The harvest are the stems of the leaves (the leaves themselves, of course, are poison). As Lazy Gardner said, it is best to cut out the flower stems as soon as you see them. You can pick the leaf stems as soon as they are a decent size. Do let the plant keep some leaves to carry on photosynthesis. The old leaves, as Lazy Gardner mentioned, get pithy, tough or hollow. The stem color has no bearing on readiness for harvest, some varieties have red stems and others have green. Even on the same plant, the color of stems will vary, but that will not effect taste. Good picking!