I think shared care is definitely harder as they get older - because parenting young children is mainly about being the person in charge of them - so either that's not you (because their dad is around) or it clearly is you, because you've been left in charge for a bit. And when they're back with their other parent, then it's definitely not you.
But parenting teens is much more about building the relationship that you can hope to influence them - which is harder to do on any kind of timeshare basis. I find it harder parenting my own DC as teens with them being away at their dad's some of the time, unlike when they were younger, and it was very useful for him to have them for a bit to give me a break. Now there are things I need to discuss with them and they're not always around. With the DSC it's harder because I'm not necessarily the right person to have the discussions they need to have, though with the kind of shared care we have (very little leisure time with their mum) then it's not always clear who is the right person. I lot more things fall through the cracks than with younger DC.
If your DSC were with you all the time - or alternatively if they live entirely with their other parent, and just visit a bit in the holidays, then these issues wouldn't really arise I guess. But a lot of step chldren are living with some sort of shared care between two homes and I think that's harder as they get older.