Hi Kirsty
My advice would be simply - go for it!
I've been travelling with JY for several years now and every trip is an adventure, but one where you are looked after all the way and all the stress of planning and navigating round unfamiliar places is taking care of for you.
I did the Inspirational China trip last October (if you look around the various threads on the Community site you'll find a number of useful reviews and tips from previous travellers, including myself, MikeW and Bob Gray.
China is very much a 'cultural experience'. Some aspects of it - the Forbidden City, the Wall, the Warriors and the pandas and truly incredible. Others, like the...ahem...facilities, the traffic and the pollution, are pretty challenging at times but all part of the experience.
One tip I would suggest if you are asthmatic or prone to allergies is to take plenty of extra inhalers/medication, because you will get through them much quicker than usual. Also take wet wipes, toilet paper, throat sweets and all the usual traveller meds such as Dioralyte etc.
Happy to help if you have any other questions. At 28, you will be at the younger end of the age range but on JY trips, age quickly becomes irrelevant as the group gels together. I once ended up Orca Watching off Vancouver Island with an 83 year old JYer and can honestly say it was the most fun I'd had in years!
Have an amazing time
Mandy