It is a difficult assessment to make. Do you start your presentation on the agreed timing for part of the audience that arrived on time or do you wait until the rest of the audience arrived?
This assessment is not easy to make and will depend on the facts and circumstances.
You may think about different strategies upfront. Was the invite for your presentation clear so there can be no confusion about timing and location? What is the percentage of your invited audience that you need to have an effective presentation? Are there any key persons that you definitely need to be part of the audience? How disturbing will it be for your audience to start a bit later? Do you have flexibility to adjust the timing of your presentation? Will it be very disturbing if people arrive and join your presentation late? If you think about these questions upfront you can design a good strategy upfront on how best to deal with the situation.