Just like other areas of the body, the breasts change with age from the 20s through the 40s...
The (usually perky) way the breasts are in the 20's are different from the way they are in the 30s, the 50s and so on, they undergo natural changes as well from the way they feel, look and general appearance where most times whoever is concerned should be able to tell (it's good to understand the whole body).
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The breast fluctuate a lot in the 20s getting bigger and smaller in between body changes especially around pregnancy and breastfeeding. The breast increase and decrease here at will according to weight gain or loss. Around this time, the nipples get darker as well.
Hormonal changes occurs around this time where lumps may become noticeable but not all may be a threat but it's safe to get them checked. The 30s also see the breasts with stretch marks from the fluctuation of the 20s as well as from pregnancy(ies).
The breast are a little dense in the 40s and it's because the breast tissue is replaced with fat at the age where there's a likely onset of menopause.
Around this time, the breast drop/sag as they won't be as perky as they used to be.