Never buy before marriage (if you expect to marry at some point). You may find that your future spouse may come from a family that has plots, or has a strong desire to eventually be buried near his/her family.
My great grandfather bought 360 plots for a family cemetery back in 1919. My wife’s father had bought plots for his immediate family here in CT, but didn’t have the foresight to provide for sons-in law or grandchildren. We’ve told my eldest SIl that she can have Mrs’ CTL’s plot for her husband if needed, Mrs. CTL will be in our family in Queens after 120 years.
As to the age 30, 40, 50, etc. It depends upon when you have settled in a community and expect to remain there long term (for those not choose to ship remains to EY for internment). OOT, shul’s generally own the cemeteries and the plot may be free with membership. My maternal grandmother convinced my aunt and uncle to buy plots in Beth David (Elmont) when Opa died. Oma is buried there, but my aunt and uncle ended up making their permanent home in Florida, when he was niftar, she bought plots there as she would not be traveling to NY to bury him or visit his grave, their children and grandchildren are in Florida. Eventually the plots in NY will become abandoned property and be used for those unable to afford plots.