Every situation (and "old" bow) is different. Myself....I would hesitate to use an old bow, especially not knowing the history, intended use, amount of use, intended draw length, etc. - especially not "knowing" a whole lot about them as seems to be your case right now. Have it inspected by someone who does know. Bows do "age", and some not so good, depending on how it was stored, and various other factors. A lot of unknowns here. Just my 2 cents, but I would make the "old" one a wallhanger, and search out a "newer" osage selfbow, if that is what you desire to try out....or better yet build one yourself. Lot's of folks "try" them, and find out it's not their cup of tea, then are ready to pass them on. Better safe than sorry.
BTW, a good rule of thumb, for selfbows or all-wood bows regarding draw length, is that un-backed bows generally are made for at least twice the drawlenght, plus 10 percent, as a minimum. So for example, if you draw 28 inches, double that would be 56....add in 6 inches and you are looking at a 62 inch bow. This is a general guideline, but a good starting point.