If you don't use the antibiotics to get rid of a UTI then will it be worse for you when your older?

Bad News. Untreated utis potentially causes damage and permanent scarring of the kidneys. I have seen several patients with renal failure, requiring a kidney transplant due to lack of treatment.It is not worth the risk to let it go. The benefits of therapy far outweigh the riskes.
UTI. Untreated UTI is never a good ideea. Not only the discomfort caused by this but also there is a chance of infection progressing into your body causing kidney problems, generalized infection. The simple UTI should always be treated:)).