this was all by design. You're too hyper focused on their average age this year and ignoring the big picture. Yes they added a bunch of older guys, but they added them all on 1 year, low risk deals that can be buried. Lucic not being in the lineup isn't luck, it's by design. Obviously they didn't expect him to go out like he did, but guys like Brown, Lucic, JVR and Shattenkirk were signed to be insurance policies in case any of the kids weren't ready, but without the dollar commitment to prevent any of the kids from taking their spot.
We're seeing that play out with Brown and Shattenkirk. Brown has been relegated to a spare part & Shattenkirk has seen repeated scratches in favor of the younger Mitchell. The team's average age going up slightly doesn't contradict Sweeney's comments about "building" around younger guys. "Building" is something you do long-term. They are actively working the youngsters into the lineup, something that is going to have impacts on their core makeup for years to come. They are building a younger core by investing into those young players. The older guys are just there as temporary solutions in clearly transitional roles. I'd be surprised if any of those guys are here next season, so I don't think it's fair to count them against the idea of building a younger core, because they are not part of the actual core.
Also, you have to consider usage. It's overly simplistic to just get hung up on average age when so many of those older guys are in fringe roles. Lucic & Brown are 4th liners. Shattenkirk is your 6th/7th D. JVR is a middle 6 winger. They replaced guys in Bergeron & Krejci who were playing big top 6 minutes. with guys like Poitras playing often at 19, Beecher playing key roles on the 4th line and the PK and guys like Zacha and JDB getting minutes that previously would have gone to Bergeron & Hall I think they've gotten practically younger for sure. Like I bet if you did an analysis of their average age but weighted it by minutes played then they've absolutely made good on their promise to get younger.