Untrained voices may have trouble with vowel formations, unruly vibrato troubles, or just lack of concept of how to blend with other voices.
Older singers may just not have the breath control to do the singing they used to. Sagging pitch or tone quality is the result.
As a conductor, you may have to do more vocal warm-ups and teaching to get the voices to adjust their vowel placement, support the tone better, and encourage staying on top of the pitches.
However, as a conductor of a small choir with older voices, you can also choose music that avoids issues that cause problems for older voices. For example, LONG SLOW Held-Out PHRASES, big variations in DYNAMICS, extremes in EXTENDED RANGES, and the need to use fancy VOCAL TECHNIQUES.
You could start by sizing up your group. If you generally have less than three in a section, avoid any music with split parts such as SSAATTBB. In fact, you may be wisest to use songs with mostly unison or two parts with occasional splits to SATB in some sections.
Be careful if you have a few great voices that overpower all the rest. Those singers may have fragile egos and will not stay if they cannot sing mostly in their natural full voices. This makes it hard to manage a blended sound in the choir.
So you may need to choose the music you perform with this situation in mind. If you have that one baritone that has a great sound but is louder than the rest of the men put together, make sure you use that to your advantage. Put all of the men on the same part as that baritone. Strength in numbers --- as well as the hope of a more blended sound.
It's all up to you as a conductor to navigate the delicate egos and enthusiastic less-trained singers you may have in your small choir. To be successful, you must keep everybody happy. Good luck!
Always have a few 2-part pieces ready to rehearse in between your more challenging pieces. Two part hymn arrangements are excellent for practicing choral techniques such as matching pitches, blending tone quality, and staying on top of the pitches. See Easy 2-Part Mixed Chorus