Before starting the regimen I used 3.75% benzoyl peroxide + clindamycin phosphate for years and didn't have much redness or drying. I recently started the regimen and found that the 2.5% BP burns my skin and turns it red. I'm not using the full amount yet either, probably around 1 pump. My skin had already acclimated to the 3.75% so I'm not sure why I have so much irritation with the 2.5%.
Also, I've read some articles/posts online that say BP can cause premature aging. Is this true? I'm a little worried about using it twice a day indefinitely if this is the case.
I'd compare the inactive ingredients. It's possible there's something in the regimen gel that you're sensitive to, or something in the 3.75% treatment you were using that has a soothing effect. I've had issues with burning/redness in the past but it seemed to be more related to using a cleanser that was too harsh and/or being too rough when drying off my face. You really have to be as gentle as possible when cleansing, drying, and applying the BP gel.
For premature aging I only have my anecdotal experience, but I've been using Benzoyl Peroxide daily for almost 25 years and I still look young for my age. I'm 36 and look pretty much the same as I did in my early 20s. I would bet on excessive dryness being a confounding factor. BP can dry out the skin and chronically dry skin can lead to cracks and wrinkles that make you look older. If prone to dry skin a good moisturizer is imperative. In my case my skin is so oily that I've never really had to worry about that.
I have found it does the opposite. Where I used BP for many years my skin is tighter and younger looking than where I didn't.
The Pro: BP may be anti-inflammatory and that could potentially slow aging.
The Con: BP eliminates antioxidants in the skin when it's applied, which could potentially accelerate aging. But...that's why you apply an antioxidant containing moisturizer after BP and you should be all set. Note that almost all moisturizers contain antioxidants to keep the product fresh.