It is most probably the age. I have grafted version of bathsheba rose in full sun for 8 hours and after 2 years it is able to handle the full sun without wilting and it has grown 8 ft tall. Make sure to apply ample mulch and water it 2-3 times a week during summer and that will help it in establishing roots.

Is this a common thing? Older, more mature roses are able to have their blossoms stand up to heat better without wilting?

Established roses have bigger roots that can gather and hold more water. It also depends on rose variety. Thick petal roses can handle sun well compared to thin petal ones.

Dirt Digger, in New Hampshire, I would think all day sun would be OK. I'll bet your rose needs more water. You could try an umbrella or chair over the rose to shade it part of the day. Don't dig it up at this point. The stress in summer would be hard on a newly planted rose--to be planted a second time. Others may disagree, I suppose. Diane

That's what I'm hoping and thinking.