@WizardOfDocs Cool.
So the first thing I'd say is: I don't expect age to twig anything for such employees. If you're worried about it, you could spin a story about needing a new suit. But a lot of folks don't get a suit or don't get one that fits worth a shit during their 20's and only decide to change that in their 30's.
Second is, the alterations that big retail places will make are often… iffy. If you feel like you have the experience and/or vision to know what you want, you can get good results by advocating for yourself and being specific. But if you just let them do the fitting and marking with no input from you, you could end up with something that fits great or that is wildly out of proportion. If you do go that route (sometimes the alterations are free), I'd say when you go back to pick it up, try it on in store and don't walk out with something you're unhappy with. It pays to be assertive and ask them to fix it.
It's often more expensive, but the way to ensure you get something that fits great is to buy the clothes and then take them to a tailor that you can establish a relationship and talk about what you're trying to do with your clothes. Like… "I want to emphasize my shoulders," by adding pads or whatever, or "I want it cut really close" to minimize bulk or whatever your goals are.
If you're lucky, this can actually be quite cheap, but that probably requires investing more time in finding the right tailor than you have.