No limits,you just need the desire and willingness to make the effort.  Though if you think your circumstances require it, getting a doctor's okay might be a good idea.  There is a 200+ pound thread on the forum, and I know that some who started running weighed 300 or more.  There are new runners of all ages.  Being older just means you have to be more careful to avoid injury as it takes longer to heal. If you are really unfit, I suggest you start by just walking.  When you can comfortably walk for three miles, then start the c25k program.  Take your time with it if you need to.  Many people repeat days or weeks as necessary.