Primarily, a player has to be comfortable when playing. For many of us, advancing age and physical limitations do change that equation. In any case, it should NEVER hurt to play.
A good strap is certainly a necessary investment.
Setting strap length to get proper positioning is also pretty important.
For my purposes, I find that instruments below 8 1/2 pounds, if they are extremely resonant, make great pizz instruments and for fretless. But, I do not care for their response playing plectrum or slap.
Generally instruments above 9 1/2 pounds are heavier than I want to deal with. Also, there is more to the mechanics on the shoulder than just the mass of the bass. Neck heavy instruments can be held in place on the shoulder with a good strap; but, that comes at the expense of torque on the shoulder that can also be uncomfortable. So, weight and balance are both important factors. In fact, just making the bass lighter may actually result in more torque, which is not necessarily a good thing. It all matters. I would be cautious with really light basses that do not balance well.
I tend to prefer my instruments to be well balanced in the range of 8 1/2 to 9 1/2 pounds. That range for me provides the tonal response I prefer while being manageable for 4 hour gigs. I also understand others may have back issues etc that may not allow. In that case, I would actually recommend getting a comfortable stool and simply sit down while performing. Nothing at all wrong with that.
