I agree with others, it depends on how premature they are. My 31 weekers were born weighing 1lb 9oz (ds) due to servere IUGR and 3lbs 1oz (dd). They will be 18 weeks corrected tomorrow and are on different pars.
DD was born needing little help, she is now in process of learning to army crawl, she smiles, giggles, grabs things, keeps eye contact and shows no signs of being premature besides her size, she's just over 10lbs.
DS however was born needing ventilation, he had 3 IVH stage 1&2. He had surgery to fix his PDA, he has a weak heart, scarred and weak lungs due to sepsis, RDS, ARDS, Pneumothorax and now BPD. He is also Pace Maker dependent due to severe Brady and irregular heartbeats. He has Mitral Regurgitation due to a leaky valve so has to be monitored and may have to have surgery to correct this. He's also a hypersensitive baby either due to all the steroids or he was born like that. He is close to home but completely different to his sister.
Your twins may be perfectly healthy or they may need help but the one thing I've learnt is that preemie are truly strong fighters :)