Such a kind and generous parental unit, that's one lucky son. First RST, Venice-Florence-Rome with my then 20 yo daughter, was a mid June departure. We had another mother-daughter pair and a family with 2 same age daughters, among members on that tour. We just did Sicily off-season last April (fantastico, highly recommend) and same as all other tours we've done during non summer time frames, my DD was youngest participant by far. My Millennial doesn't mind it, in fact loves what she thinks is a favored/special position being youngest in the group and is generally appropriately grateful just being taken on a trip to Europe. I advise going over each prospective itinerary closely to evaluate activities covered and contrast and compare with what you know your 20 yr old is drawn to. Consider pacing and length of the multi-stops; these tours provide plenty of variation of guided time and free time but packing up and moving every couple of days does mean you do not have the luxury of parking it and just being 'there' and some places are harder to leave than others. We're heading out next month on the Loire to the South of France tour and with adding nights before/after as I prefer to do, it's going to be our longest trip so far by dint of days we'll ramble on. I'm making mental preparations to achieve zen mode when inevitably our close quarters and familiarity may get the better of us, even while scarfing down as much fromage, baguette and vin as we can possibly manage. I'm in the camp where I expect any travel partner of mine may well get on my nerves at some point of a lengthy journey and simply try best to keep things light when crankiness rears its frustrating head. We're not joined at the hip, we take solo time at different junctures and that works for us. I've always asked my gal to do her own research/provide input on additional treks/alternate activities we could/would do while we're away. She's been spot on many, many times and it's added a lot to our travels. I consider myself lucky, she has a knack of quickly getting physically/geographically oriented once landed in a new spot and I have benefited greatly from following her navigational lead. Happy family travels to you and yours wherever you roam.