You can absolutely adjust acidity in a wine when it is one year old. Though I often say that it’s best to do major adjustments early on in a wine’s life (since the additions will have time to integrate better) it’s always possible to make tweaks further down the road if you think it makes the wine better. Actually, I think a little tartaric bump, maybe around 1 g/L or so, will really benefit the long-term aging and microbial and color stability of your wine. A pH of 3.8 is a little bit high and if you can get it down just a tad you’ll find your free SO2 will stick around longer, your chances of a runaway VA (volatile acidity) will lessen and your Merlot’s color might be preserved longer.
As always, I recommend “bench trials” (small-scale additions to a series of 100 mL lab samples of your wine) so that you can taste and find that sweet spot. Adding a touch of acid might renew the “zing” and really perk up the wine from a taste bud sensory point of view.