Does waiting a few years before pursuing a PhD affect chances of getting admitted? This is a somewhat long question. I hope you can take the time to read it carefully.
I obtained my bachelor's degree in physics in June 2019. I decided I wanted to go to physics graduate school in the USA after finishing my undergraduate degree, however I noticed I was not going to be a strong candidate because I only had one letter of recommendation, had very little experience doing research, and had never published a paper. Hence, I decided to postpone my application and work with physicists to do research. Since I come from a place where research opportunities are scarce, I decided to contact researchers from the USA to work with them. It was really hard to get a response from them, but finally I got a response from two very good physicists from UC Santa Cruz and Georgetown. I am currently working with one of them and there's a possibility to publish an article by the end of this year's  Fall. The other physicist advised me not to work with the two of them at the same time, as I might become overwhelmed by the amount of work from both projects. So he told me that it was my decision if I wanted to apply this year or wait until the next year. He said to me that if I waited one more year to apply, we would work together on a project and he claimed that we would publish a paper together by the end of Fall 2021.
My dilemma is that if I wait one more year I would be older and when I start my phd, three years will have passed since I graduated from college. However, if I wait one more year I will have one more letter of recommendation and one more paper published (which would probably make me a stronger candidate).
My question is the following: is it a better idea to wait one more year considering that I can get all the things I mentioned before? Or, does waiting this amount of time affect my chances of being admitted?
I graduated from a 4-year physics program at the age of 22 (I'm 23 now).
Waiting can negatively affect your chances of admission if you aren't doing academia-related things: for example, if you quit to join industry for 3 years, then when you get around to applying your letter writers might not remember you that well anymore (they might even have died/retired/resigned), which will have a negative impact.
What you are doing, on the other hand, will not negatively affect your chances - in fact you are likely to improve your chances. You are after all getting more/better letters of recommendations &amp; publications, two key aspects to an application.