I've been to Saariselka with friends (all of us are late 30's) and we found that there was plenty to do. We booked a 7 night package through Inghams and stayed in a log cabin. While we were there, we choose the learn to cross country package that gave us instruction for 3 mornings and hire of the equipment for the total duration, so that took up most mornings. We went out with the husky dogs, which is well worth doing, and we went on a reindeer trip too, which was pleasant. We took skidoos up the fell one evening to see if we could see the northern lights (no joy for us!) These we'd prebooked with inghams before we left as I'd looked online prior and found the prices where comparative with booking locally.
We also messed about on the tobogan run, (free) if you buy ski lift passes you can go from the top, but we choose to walk up the side of the run, and then when we were to tired, we'd fling ourselves back to the bottom! most of the things we did where in the afternoons. There's some lovely little cafes around Saariselkas' centre to get hot drinks in after spending all morning outside. We also paid to use the pool and slides in the Holiday Club Hotel and Spa, which was great to warm up in. On one of the days we took a very long cross country ski over to another hotel,
http://www.suomenlatu.fi/kiilopaa/en/smoke-sauna/
where they had a traditional smoke sauna, which was an experience well worth doing.....
we wanted to try ice driving/karting, here:
http://actionpark.fi/index.php?option=com_content&view=article&id=51&Itemid=28&lang=en
but we ran out of time...
so personnally, I'd say there's loads to do during the day, and it doesn't matter what age you are, do them! In the evening we entertainied ourselves with cooking, board games, films, building fires, jumping around in the snow after being in the sauna, and making ice glasses for vodka, (this didn't work too well)!
So I'd say pre-book the things you'd definetly like to do huskys/reindeer/skidoos/ski-ng through a company like Inghams. We found their prices to be reasonable, but anything out of the ordinary like smoke saunas/ice driving/watching films outdoors, arrange these for yourself prior to going, or at least know where to ask once you arrive...hope this helps!