Don't call every older person 老大: A common mistake for learners is to use 老大 to address any person who is older than them. This is incorrect. 老大 implies a specific hierarchical relationship within a defined group (your family, your company, your friends). Calling a random elderly stranger 老大 would be confusing;
大爷 (dàye) or
叔叔 (shūshu) would be more appropriate.