This article here says exactly what I'm thinking. To me, frugality is about being efficient, which means you spend the right amount of energy, time and money without wastage. And efficiency varies according to the different life stage that you're in.
For example, when I was a student with very little pocket money to spare, I'll go all the way to some cheap bargain sales at Peninsula Plaza to buy a pair of track shoes. There are many shops there that are in close proximity to each other, and you can bargain there, so it'll be easier to find something good and really affordable. But the downside is that to save that $20-$30 bucks, you might have to spend 1 to 3 hrs there. While I would certainly do that in the past, I wouldn't care to do it now. What had changed? My time became more precious, so spending 1 to 3 hrs to save $20 to $30 bucks is not what I would do now but is something I would gladly do in the past.
Efficiency can be seen as a percentage also. If I have $200 in my bank account, saving $20 is a big deal to me because that's easily 10% of my networth. If I have $200,000 in my bank account, saving $20 is just a drop of my networth - a mere 0.01%. No way am I going great length to save 0.01% of my networth, especially if it takes up a lot of my time. This can be easily applied to our free time too. During my student phase, there's school holidays, so I'm practically free for 15 hours per day, every day, for 1 month (not much homework in the past). That's a freaking 450 hours of free time. If I have to queue for 5 hours for a freebie, why not? It's just about 1% of my free time. However, when I'm working, and I only have 24 hours of weekend time free, spending 5 hours will take up a proportionally bigger 20% of my time. As a full time tutor, especially now during the exam season, I really only have about 8 hours of free time per week. Spending 5 hours to queue up? No freaking way. I'll rather spend the money and use the time to sleep.
So, time is money. When we're young, our time is cheap and we don't have a lot of money. Hence, we spend time to save money. When we're older, (hopefully) our time becomes more costly, and we have a lot of money. Therefore, we spend money to save time.
But habits can be hard to correct. Imagine a big part of your life you've been trying to spend time to save money. The cost of your time might have increased tremendously but you're still stuck to the habit of using time to save money. That would not be wise and efficient, because while you're a good saver of money, you're not a good saver of time.