Not too old at all
We adopted Kailee when Buddy was 7 years of age, and even though they only had 7 short months together, they were good months. Kailee was 2 at the time, and given her young age, she had a ton of spunk that seemed to liven up a more retired acting boxer

Buddy spent his last months living like a puppy again, and enjoying each day he had with her.
Now, we have adopted Clyde who turns two in June. Kailee is 4 1/2 years of age, had just over two years of being the only furchild, and this too has gone well. There have been zero issues between them, they enjoy each others company, and play together pretty much all day.
We too had no skin kids during this time, Buddy was the centre of our lives, as was Kailee for the past two years, but both accepted a new fursibling without any problems. I believe if your present boxer is secure in the relationship they have with you, they are not threatened by a newly adopted sibling. I say go for it :D
Jean