Typically it's somewhat to be expected. SHBG decreases with age for all men, which isn't really a bad thing. When having your total Testosterone tested this also gets pulled in, which is why 400 for a 20 year old and 400 for a 45 year old would likely mean you actually have more free Testosterone than that 20 year old.

It's not something that's worth treating itself in almost all TRT cases, as the biggest benefit you get from TRT comes from additional free Testosterone. Raising that will almost always give you the results you were looking for despite any low SHBG.

DIM as an aromatase inhibitor and estrogen management supplement shows great promise. We know that DIM can reduce the more harmful forms of estrogen (E1 and E2) and increase the more beneficial form (E3). It reduces aromatization. However, studies are limited so we don’t really know how well it works at doing these things. It is currently sold as a supplement, so it is not regulated in any way, meaning you may not actually be purchasing DIM when you order it online.

TL;DR: If you can find a trusted source, and the price is right, DIM appears to be a safe way to manage estrogen levels.

Low SHBG is generally considered desirable as it leaves you with more bioavailable testosterone to actually attach to androgen receptors. SHBG lowers from age 20 until about age 60, at which point it begins to increase. At age 45, you should be at the lower range of normal.