you can see from the above posts that you will not be the only one practicing aikido with a back problem. I had one before I started, and still have one now many years later, however, I have only missed a few practices in that time. I feel that continuous practice has helped keep me flexible and in recent years, now I have learned to move completely from my centre, helped protect my back and from making matters worse.
Aikido exercises performed poorly and with uneccessary tension can put strain on the lower back in particular. So always err on the side of caution, learn to roll correctly. High/hard breakfalls do not have to be engaged in until you are ready ( IMHO they are generally to no-ones benefit - apart from making the thrower look good
If you have a choice of aikido clubs in your area, watch them, notice how the teacher handles the students that may not be young/fit/athletic (there will definitely be some). Notice how the students practice, is there respect for the limitations of the less able?
Anyway, my advice fwiw is, start, take care, enjoy the journey, you can always get off the aikido bus whenever you choose