Using tickets with a senior discount

That’s a pretty broad question since every place might have its own rules. Where exactly are you planning to go?

I can tell you how it usually works around my area—Barcelona:

SKIP-THE-LINE
The term “skip the line” is a bit misleading, because in most cases you’re not really skipping anything. While at most regular sites there are usually two lines: one to buy the ticket (at the booth or a machine), and another one to actually go in, the most popular attractions don’t even have that first line anymore, because tickets are only sold online.

Some intermediaries play around with the term “skip the line”, which is pretty dishonest in my opinion. They’ll tell you they’re selling you an online ticket that lets you skip the line—but in reality, you can buy that same ticket directly from the official website of the place you want to visit, and avoid the extra commission these intermediaries charge. Hence, the reason I always advise purchasing tickets on the official sites, not through intermediaries. It’s misleading—they’re pretending to offer a special service when they’re really not.

Purchasing online will save you the potential lines on-site at the booth/machines -but again, the most popular sites don't sell on-site anymore, only online.

TIME SLOTS
Also, when you buy your ticket for these popular sites, you have to pick a time slot at which you will visit. Then, you have a margin of 15 to 30 minutes —depending on the site- to go inside the site. This helps control the number of people inside and makes sure it’s not too crowded. That way, visitors don’t feel overwhelmed by the number of people inside.

So for example, if you book a visit to the Sagrada Família at 10:00 AM, that means you can go in anytime between 10:00 and 10:15 (or maybe 10:30, depending on the place). If you show up later, they will not let you in and you’ll lose your ticket. Once you’re inside, you can stay as long as you want, but keep in mind that many places are designed like IKEA stores—with a one-way path that slowly guides people forward. You can go at your own pace, but always moving ahead.

Now, some places—like Casa Batlló—really go all in trying to squeeze money out of visitors. They offer a range of tickets, including an expensive "VIP" one that lets you enter whenever you want, without a fixed time slot. With that kind of ticket, when you show up, you just get in line with the people entering in the next group (they usually let people in every 30 minutes).

All of this mostly applies to the most popular attractions. For the rest, you can usually just buy your ticket at the booth when you get there (or online, if you prefer), and walk right in.

SENIORS
As per senior priority, I am afraid not, in most of Europe we're pretty much all for the ideals of the French Revolution: liberté, égalité, fraternité... Only disabled people (ie wheelchair) might have some kind of preferential treatment when visiting -and only at certain sites.

Since you bought the tickets online, you still have to show proof of age, so don't forget to carry your passports with you or official ID card if your country issues them.
So yes, in Madrid we had several senior prices. Since I bought the Prado online, we were directed to the special ticket office to pick up our tickets, and then stood in the regular line for ticket holders to enter. No golden ticket in those cases.
Elsewhere, we had a special entry line for old people, errr I mean seniors, that gave us immediate access to the ticket office and then on into the place. Everywhere we showed our official ID to justify the reduced price.

We were in Spain earlier this spring and I bought tickets online before we left. Not once did we have to show ID, but we always had our passports with us in case. With buying tickets in advance, you skip the long ticket line, but you still wait in line with everyone else to enter places.

Do not count on getting into the high-traffic sights right at the time shown on your ticket unless you have an early-morning entry time. As Enric wrote, some are very crowded. In my experience, there was sometimes a door-minder paying attention to how many people were leaving as new people were allowed to enter.

Thanks for the info. I guess we will figure it out as we go along!