Lily and Marshall's son Marvin is growing up so fast! On March 6 Carter Matt reports that not only is he growing up quickly, but casting for a Marvin in his 20s is taking place for an upcoming episode that will be partly dream sequence/ flash forward.
With a family like the one that Marvin is growing up in, it will definitely be interesting to see how all these quirky characters that make up his family will have an effect on him and how he turns out.
There is no mention who the dream sequence featuring an older Marvin is coming from, so it is likely to be hilarious and full of crazy stories. If it's Marshall's story, worry could be an underlying tone of the episode as he tends to over think things and make a big deal from nothing.
The episode is entitled "Romeward Bound" and will be the 22nd episode of the season. It will take place before we get to see Barney and Robin marry. Are you interested to see how Marvin turns out?